A HISTORY OF RAILWAY SIGNALLING
(from the Bobby to the Balise)
Stephen Clark*
*Lloyd’s Register Rail, UK
71 Fenchurch Street, London EC3M 4BS
Email: stephen.clark@lr.org
Keywords: History, operations, technology, signalling.
Abstract
The paper outlines the history of signalling from the opening
of the first purpose-built passenger-carrying railway in 1830
with hand signals, through the developments of fixed lineside
signals, electric telegraphs and interlocking mechanisms for
points and signals. From the appearance of power signalling
at the turn of the 20th century, it follows the development of
first electrical and then electronic signalling technology
through to present day communication-based systems.
1 Preamble
The following chapters present an outline history of Railway
Signalling. Although the basic principles of railway signalling
and control are universal, the way in which signalling has
developed in Britain differs in a number of details from
practices used in Continental Europe and America. What is
described here should more properly be called a ‘Brief
History of British Main Line Signalling’, and covers
developments in both Great Britain and Ireland.
2 Introduction
It is likely that the earliest railways existed in or about mine
workings – the concept of a prepared way would have been
most useful in conditions unsuitable for ordinary wagons or
pack-horses, where heavy loads in quantity would have to be
carried over rough ground from a mine to a road, or a canal.
In Britain, the earliest record of a ‘waggonway’, using
wooden rails, dates from 1630, when one was laid down near
what would become the cradle of both railways and coalmining, Newcastle upon Tyne.
Track using plain wooden rails quickly wore out and evolved
into the ‘plateway’ with iron plates fixed to wooden bearers
(track maintenance workers often used to be referred to as
‘platelayers’). Cast iron rails fastened to wooden sleepers in
the now familiar pattern first appeared at the Duke of
Newcastle’s Colliery near Sheffield in 1776, and the
Middleton Colliery Railway, constructed to transport coal to
Leeds, was the scene in 1812 of the first recorded commercial
use of steam locomotives.
In North-East England, the much-celebrated Stockton and
Darlington Railway opened in 1825 as a freight-carrying
railway, using both rope and locomotive haulage for its goods
traffic, although passenger coaches pulled by horses were
provided later as an afterthought.
3 Origins of the railway
In September 1830, the Duke of Wellington opened the
Liverpool and Manchester Railway, the world’s first purposebuilt passenger-carrying railway with haulage by
locomotives. The opening of the L&MR can therefore be
considered as marking the beginning of the ‘Railway
Signalling Age’, a good point at which to start a survey of
signalling and its development. There is an irony in the fact
that its opening is now remembered not so much as the dawn
of the railway era but because there occurred during the
celebrations Britain’s first public railway accident, in which
the local MP, William Huskisson, was run down and severely
injured by a train hauled by the ‘Rocket’, driven by George
Stephenson. Despite Stephenson himself driving a special
train conveying the unfortunate man to obtain medical
attention, at a speed reported to be nearly 40 miles per hour,
Huskisson died later the same day.
To give a History of Railway Signalling some sort of context,
it is useful to start with a summary of the history of ‘the
railway’ itself.
One can well imagine Stephenson’s feelings as Huskisson
stumbled into the Rocket’s path, but consideration of the
reasons why he was unable to stop in time brings us back to
the theme of Signalling and its development.
The railway, by which we mean vehicles with flanged metal
wheels running on a guideway of metal rails, has been in
existence for less than 250 years. It has proved to be the most
efficient form of land transport, in the sense of being able to
move heavy loads at high speeds over long distances, yet
devised. It is, however, the qualities that give the railway its
efficiency and the ability to move heavy loads along a lowfriction bearing surface that create the need for the system of
signalling and control that we will look at in this paper. For
the moment, consider your own experiences as a car driver,
and think about the ability to stop quickly from various
speeds – we shall return to this theme later on.
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4 The Beginnings of Signalling
Those of us used to driving cars are familiar with the concept
of ‘stopping distance’. To stop a car travelling at speed
requires a distance proportional to that speed. The Highway
Code tells us that to stop from 30 mph, even with the high
level of friction available between rubber tyres and a dry,
well-maintained road surface, will require 23 metres or 75
feet, and under the same conditions from 60 mph, not twice
but over three times as far, 73 metres or 240 feet.
But for a train rolling on steel wheels along a guideway of
steel rails, levels of friction, and hence adhesion, are much
reduced. In the case of a modern passenger train such as the
diesel-powered ‘Inter-City 125’, the distance required to stop
from its maximum speed of 200 km/h (125 mph) is nearly one
and a quarter miles, even with superior brakes. This is not an
unreasonable comparison; back at the dawn of the railway
age, the problem uppermost in the minds of engineers and
operators was how to keep them going rather than how to get
them to stop safely. The rudimentary braking technology
available at that time would not stop a train travelling at 50
mph on the level in much less than three-quarters of a mile.
So, with trains hauled by steam locomotives that could reach
speeds of 50 mph or more, you could not rely on a driver who
saw an obstruction ahead being able to brake sufficiently hard
to avoid colliding with it. Neither could he steer out of the
way, so there arose the need for new disciplines that would
ensure a safe separation of moving trains by means of signals
to drivers from the lineside.
in a section, out of sight between two policemen, a member of
the train crew (usually the guard) had to run back along the
line as far as possible to show a hand signal when the next
train approached. Given the poor efficiency of train brakes in
those early days, this would have required a run of at least
half a mile, or more if time allowed.
In the absence of physical safety devices, signalling at the
dawn of the modern railway age depended on a detailed code
of rules, procedures and instructions that the railway’s
servants were expected to follow with military discipline.
Where mechanical failures led to accidents this was often due
as much to lack of experience of where the system might fail
as to the failure itself.
Incidentally, when a modern train driver doesn’t know a
signalman’s name, he will often address him as ‘Bobby’, a
reminder of his railway policeman ancestry.
5 Fixed Signals
Although Time Interval working remained in widespread use
up until the 1860s, fixed lineside signals began to appear as
an alternative to the policemen’s hand or flag signals in the
late 1830s. At first these simply mimicked hand signals on a
larger scale, with arrangements of moveable flags or discs and
coloured lights being mounted on tall posts and operated by a
policeman, but had the advantage of being visible at greater
distances. Signals were displayed in accordance with the
convention that Red indicated ‘Stop’, Green ‘Caution’ and
White ‘Clear’.
Before proceeding, it is worth remembering that at this time
none of the facilities regarded today as essential to safe
operation existed:
x
no telegraphs, telephones, or other form of
instant communication;
x no lineside signals;
x no brakes at all on the majority of vehicles;
x no centralised control of points;
x no whistles on locomotives until 1833.
The first signalling systems were therefore entirely humanbased, the line being divided into sections of approximately
two miles with hand signals being given to train drivers by
Railway Policemen stationed at the beginning of each section.
A policeman would indicate a clear way ahead by standing
facing an oncoming train with his arm outstretched. After a
train passed him and entered the section he would assume a
‘stand at ease’ position. He would continue to signal an
obstruction, if another train approached his position, until a
time interval (typically 7 – 8 minutes) had elapsed, after
which he could permit a following train to proceed, but under
caution. In this way train separation was maintained, and to
allow policemen to impose consistent time intervals, the
railway company would issue them with sand glasses or ‘egg
boilers’.
The Time Interval system of signalling did have one
insurmountable drawback. If a train broke down and stopped
Figure 1: Great Western Railway ‘Disc and Crossbar’ signal
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Adoption of a white light for ‘clear’ seems odd to us
nowadays, but in the early to mid-19th century, before the
widespread use of gas (and later electricity) to light houses,
roads and public spaces, the countryside at night was
profoundly dark, and there was little chance of confusion
between signals and external lights.
What also appears as odd to later generations is the accepted
practice of a signal conveying ‘Clear’ or ‘Proceed’ simply by
the absence of a Danger indication. Red flags or discs would
be turned edge-on to allow a train to proceed, and in the most
famous example, a ball signal was displayed at the approach
to Reading Station on the Great Western Railway and
described in that railway’s Regulations thus: ‘A Signal Ball
will be seen at the entrance to Reading Station when the Line
is right for the Train to go in. If the Ball is not visible the
Train must not pass it’. It was not until the late 1870s that a
serious accident called this arrangement into question and the
practice was changed.
In 1841 Charles Hutton Gregory designed a Semaphore signal
to be used on the London & Croydon Railway, and the result
was the first example of what we would now recognise as a
‘railway signal’. Although the signal was fixed in position
alongside the track, it still needed a man there to operate it. In
1843, Gregory built a contraption of levers and stirrups, by
which a number of signals and points could be operated by
one man from a central location, together with a very basic
form of ‘interlocking’ to prevent a signalman from operating
points or signals in a way that could lead to a derailment or a
collision. This was however extremely crude, and didn’t
provide what we would now understand as true interlocking
whereby one lever movement must be completed to allow
another to be moved, a development that would not appear
until the 1860s.
6 The Coming of the Telegraph
Despite the advances in lineside signal design, the method of
keeping following trains apart by time interval working
continued. What was needed was a simple and reliable form
of communication between the policeman at one end of a
section and his colleague at the other that would allow trains
to be operated according to a system of ‘space interval’
working rather than the fragile protection offered by the
policeman’s sand-glass. The device that would provide this
communication and start the long and intricate story of
electrical railway safety devices was the Cooke & Wheatstone
electric telegraph, first demonstrated in 1837.
Early telegraph instruments used a pointer or ‘needle’ that
could be moved to the left or right to allow messages to be
sent by spelling out words letter-by-letter using a telegraphic
code (of which the American Morse Code was only one of
many). In the railway application this allowed a policeman to
report a train entering the section to his colleague down the
line, who could in turn could report back when it left the
section. If the train didn’t arrive, or arrived incomplete, and
no report was received, any following train would be stopped
and detained.
Figure 2: Block Telegraph instrument
In this way, a system of signalling in which the whole of a
train entering a ‘block section’ must be positively observed to
have left it before another train can be admitted was a
practical possibility. This system came to be called ‘Absolute
Block’ and began, slowly, to be adopted by the more
responsible railway companies. For example, by 1852, the
forward-thinking Great Western Railway had installed
lineside wires on all its main routes for the electric telegraph.
The photograph shows a typical Block Instrument that
evolved out of the experiments and problems with the original
telegraph instruments. It is capable of showing three
indications – ‘Line Blocked’ (the normal condition of the
section), ‘Line Clear’ and ‘Train On Line’, and in conjunction
with a single stroke bell for exchanging coded messages,
provides the basis for the Absolute Block system that
eventually controlled train movements throughout the British
railway network.
A useful spin-off of the spread of railway telegraphs across
Britain was the introduction of ‘Standard’ or ‘Railway’ time
(the concept of Greenwich Mean Time wasn’t introduced
until 1880). At a time when ‘Bristol’ time was some 15
minutes later than ‘London’ time, this was in itself a minor
social revolution.
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7 A Digression - Railway Braking systems
Although strictly outside the scope of a study of signalling,
mention must be made of railway brakes, without which a
train cannot be controlled and any signalling system is of little
or no use.
Early train brakes were primitive in the extreme; a
mechanical brake being provided on the locomotive or its
tender, and a similar arrangement on a brake van at the rear
of the train. Because there were no brakes at all on the
wagons or passenger coaches in between, stopping and
starting a train required a fine degree of co-ordination
between driver and guard (the driver would use the
locomotive whistle to convey his instructions) to synchronise
the braking being applied and avoid the train being squeezed,
or even worse, stretched, and couplings broken.
Throughout most of the mid-Victorian era from 1840 to 1890,
railway engineers sought to devise means of providing
‘continuous brakes’ which would act on all vehicles
throughout a train. Most of these were unsuccessful, some
spectacularly so, but in the absence of legislation, the railway
companies kept experimenting with systems using rods,
chains, hydraulic, steam and air pressure, and vacuum. It took
a truly horrific accident to force the Government to make
continuous brakes, acting on every vehicle and automatically
applied in the event of vehicles becoming inadvertently
detached from a train, a legal requirement.
8 A Word about Accidents
At the beginning of the 21st century we are accustomed to the
concept of ‘engineering for safety’, where design and
implementation of complex systems such as aircraft or
industrial plants whose failure can have serious – or, in the
case of nuclear facilities, unimaginable – consequences are
subjected to rigorous processes of review and analysis
throughout their lifecycles to identify, record, and control all
possible hazards.
Development, adoption and use of these processes has been
very largely dependent on experience and understanding of
systems and their behaviour. In the middle years of the 19th
century, however, with railway engineering and safety
disciplines still in their infancy, there was little or no such
experience on which to draw; the evolution of safety was
slow and mostly reactive, a major accident frequently acting
as the incentive to improve equipment, rules or practices.
Legal regulation of the early railways was surprisingly
limited, the best description of the Government’s philosophy
being ‘supervision without interference’. An Act of
Parliament passed in 1840 allowed the Board of Trade (the
Government’s economic advisory committee) to appoint
Railway Inspecting Officers. These were serving officers
recruited from the Army’s Corps of Royal Engineers with
powers to inspect and report on new railways and approve
their opening for public use, and to investigate the causes of
railway accidents.
Unfortunately, it was very seldom the case that innovation in
the field of railway signalling was followed by a headlong
rush by railway companies to implement the new technology.
Many of the devices and systems that we now think of as
providing undeniable safety benefits were available for many
years before companies would agree to install them, either
because of the costs involved, or often because they had been
developed by and used on ‘another Company’s railway’. For
example, slow take-up of the telegraph block system was in
no small way due to the stubbornness and, one might be
forgiven for saying, arrogance, of railway company directors,
an attitude clearly demonstrated by the Company Secretary of
the London, Brighton & South Coast Railway in a muchquoted reply to the Board of Trade regarding the Inspecting
Officer’s report into a serious accident in Clayton Tunnel in
1861:
“My Board feel bound to state frankly that they have
not seen reason to alter the views which they have so
long entertained on this subject, and they still fear that
the telegraphic system of working recommended by
the Board of Trade will, by transferring much
responsibility from the engine drivers augment rather
than diminish the risk of accidents”.
Note those words - ‘Recommended by the Board of Trade’ –
it was to be nearly 30 years before a particularly catastrophic
accident forced the Government to give legal powers to the
Board of Trade to not just recommend, but demand, the
adoption of basic safety systems on passenger railways. In the
meantime, Inspecting Officers continued to investigate every
accident, recommending in one report after another adoption
of the three basic safeguards of railway safety, namely
interlocking of signals and points, Absolute Block working
and continuous automatic braking systems on passenger
trains, frequently shortened to the memorably monosyllabic
‘Lock, Block and Brake’.
The history of railway signalling in the Victorian era is thus
linked closely with that of accidents. For further insights
readers are encouraged to obtain and read the original and
arguably the best study of British railway accidents, ‘Red For
Danger’ by L. T. C. Rolt, first published in 1955 and
reprinted and updated a number of times since then.
9 The Lighter Side - Other Safety Devices
Reading contemporary newspapers from the Victorian era,
one can feel a sense of déjà vu reading journalists’ and
correspondents’ criticism and condemnation of the railway
companies for their lack of humanity in the treatment of staff
(24 hour shifts were not uncommon) and apparent pursuit of
profit at the expense of safety. One of the most caustic critics
of the railway industry was the magazine Punch, which first
appeared in 1841 and relentlessly pursued and lampooned the
railways well into the 20th century.
9
To an extent that seems extraordinary to us today, endless
curious suggestions were sent to newspapers, and even to the
railway companies themselves, including a demand that all
passengers should be issued with inflatable suits to protect
them in the event of a collision.
One of the most imaginative and at the same time least
practical was the suggestion made in a letter to the Chairman
of the London & South Western Railway, also in 1841. This
required sacks of wool to be suspended at the front and rear of
the train and in between the coaches so that in the event of a
collision, the energy would be absorbed by the woolsacks and
the passengers would thus come to no harm.
10 The late Victorian era
Returning to the timeline of signalling development, Britain’s
railways had by the 1870s voluntarily adopted some if not all
of the triple safeguards of Lock, Block and Brake. Once the
issues of communication along the railway to allow Absolute
Block operation, and of giving consistent signal indications to
drivers had been sorted out, there remained the important
consideration of how to apply signalling safely to the ever
more complex station and junction layouts that were
developing.
Semaphore signals were by this time in widespread use to
control train movements, being operated mechanically from
interlocked lever frames in signal boxes. The development of
mechanical interlocking provides a good example of the flood
of railway engineering innovations appearing in the later
years of the Victorian era. From the 1860s, the complexity of
stations and junctions had provided the incentive for
development of more reliable and positive methods of
interlocking. A host of inventions appeared, with a number of
different companies seeking to patent their particular method
of interlocking and then convince the railway companies of
its virtues.
By the 1890s, some 40 different methods of interlocking – all
achieving the same end - had been designed, built and
patented. There were lever frames using tappets, grids,
rockers, tumblers, cams and studs. There were frames with
locking actuated by rocking shafts, hook racks, soldiers and
twist bars, and interlocking composed of dogs, darts, half
locks, swingers and diamonds.
Mechanically interlocked lever frames reached a peak of
complexity and size in the closing years of the 19th century,
the record being set by a frame containing a single continuous
row of 295 levers in a signal box at York. Some of the
interlocking mechanisms were of quite astonishing ingenuity,
but were at the same time extremely difficult to maintain - the
designs which survived the longest therefore tended to be the
simplest and most robust. Even today, over a dozen different
designs are still in use in Network Rail’s several hundred
mechanical signal boxes.
The one area where the march of safety had been slow or
stationary was the adoption of continuous brakes. Mention
has already been made of the proliferation of braking systems,
with advantages being claimed for each by their supporters. In
1875 a trial of 11 different braking systems had taken place at
Newark, which eventually demonstrated clear superiority of
the automatic vacuum brake.
It was not until 1889 that the issue would finally be decided in
a shockingly dramatic and tragic way.
11 Armagh 1889 – an ‘Event Catalyst’
On 12 June 1889, an excursion train of the Great Northern
Railway of Ireland left Armagh for the seaside town of
Warrenpoint, on the shore of Carlingford Lough. Nearly a
thousand passengers, over half of them Sunday-school
children out on a treat, were packed into 15 coaches, and as
was the usual practice with such excursions, the coach doors
were locked.
It had been expected that the train would be made up of only
13 coaches, that the locomotive to be provided would be
adequate to the task of taking the train over the steeply-graded
line, and that an experienced driver would be available. All of
these expectations proved to be wrong. Two extra coaches
were added shortly before departure, the locomotive was thus
inadequate to its task, and the only driver available had never
driven a train over the line before.
Barely two miles from Armagh, the locomotive stalled on a 1
in 75 gradient through lack of steam. After some discussion
between the train crew, the decision was taken to divide the
train to allow the loco to reach the summit with the first five
of the 15 coaches.
What made an already bad situation worse was that the train
was fitted with an early, non-automatic system of continuous
braking called the ‘Simple Vacuum’, a pernicious and
potentially lethal system in which vacuum created by the
locomotive was piped through the train and used to apply the
brakes. By contrast, in the automatic vacuum brake, which
was by then already used by a number of British railways, and
remained in use until the late 20th century, the vacuum holds
the brakes off. Atmospheric pressure is used to apply the
brakes when air is admitted to the brake pipe through the
driver’s brake valve, or through any opening in the pipe, as
when a vehicle becomes detached.
It was equally unfortunate that the line was still being worked
under the time interval system, and that the train crew,
occupied with dividing the train, had neglected to take any
steps to protect it.
Although they must have realised the potential danger of their
course of action, the train was divided between the fifth and
sixth coaches, the Simple Vacuum brake thereby becoming
totally ineffective with the only brake remaining to hold the
train on the gradient being in the van at the rear. And,
inevitably, when the locomotive tried to restart on the
gradient, it set back slightly and bumped the detached
10
coaches, pushing them over the stones which the crew had
placed under the wheels in a futile attempt to hold them on
the gradient. The coaches then ran away down the 1 in 75
gradient gathering speed, and collided with the following
train (which had been authorised to leave Armagh under the
time interval rules) at over 40 mph, completely destroying the
last three vehicles, killing 78 passengers (a third of them
children) and injuring a further 260.
This disaster, which eclipsed all previous British railway
accidents in its scale and casualties (including the collapse of
the Tay Bridge in 1879), proved to be a defining moment in
British railway and signalling history. Public opinion was so
outraged that the British Government was compelled to pass –
less than three months later - the Regulation of Railways Act
1889 which finally gave the Board of Trade’s Railway
Inspectorate legal powers to compel railway companies to
provide the following safeguards:
The other signalling technology development of major
importance during the 1890s was the widespread introduction
of the electric token system of single-line working invented in
the 1870s by Edward Tyer. Now, instead of the old - and
ultimately fallible – human methods using telegraphic orders
or staffs and tickets, the issuing of a single line token – the
visible authority for a driver to enter a single-line section could be electrically controlled so that one could be issued at
either end of a section, wherever it was needed. With this
method, the need for hand-written tickets or orders, and the
hazardous work-arounds that had been required when, for
example, an operating problem left a train at one end of the
section and the token at the other, became things of the past.
x
to install interlocking between points and
signals;
x to adopt the Absolute Block system to control
trains (rather than the time interval method);
x to fit continuous, and automatic, brakes to all
passenger trains.
Thus did ‘Lock, Block and Brake’, the basic pillars of
present-day railway safety, at last become law. Subsequent
technical development, as we shall see, tended more towards
increasing efficiency and eliminating human error, although
for many years this would be far more successful in
protecting signalmen, rather than drivers, from the
consequences of their mistakes.
We can therefore consider the modern signalling era as
starting from this point.
12 Signalling and Operation - developments
after 1890
Following the 1899 Regulation of Railways Act, British
railway signalling entered a period of renewed development,
invention and innovation. Now that the foundations of
modern signalling practice had not only been laid but firmly
cemented in place by statute, the way was opened for further
improvements, both technical and procedural.
As adoption of Absolute Block working was now no longer a
matter of choice, there was a proliferation of styles and types
of instruments, and methods of operation, as we have already
seen with lever frames and interlocking. Two-position and
three-position block, one-wire and three-wire instruments,
rotary and sequential block - all had their adherents. One of
the most successful and widely-used of the ‘enhanced’ block
systems was Sykes Lock & Block. This not only integrated
the exchanging of messages between signalmen at the ends of
a block section into the operation of the block instruments but
also mechanically interlocked the signals with the block
working and the block working with the passage of the trains
through the section. This made operation by manual block
methods as near foolproof as it could be.
Figure 3: Tyer’s Single Line Key Token Instrument
As with other signalling developments, a basic idea would
often be developed in various ways by different railway
companies or manufacturers. Single line instruments took
many forms, and the ‘tokens’ came in all shapes and sizes
including tablets, staffs and keys. The photograph shows a
Tyer’s Key Token instrument, many of which are still in use
on Network Rail and British heritage railways.
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13 The First Power Signalling Systems
By the end of the 19th century, the use of electricity in the
service of railways already had a firm foundation – Volk’s
Electric Railway in Brighton, the first electric line in Britain,
opened in 1883, and the first of the deep-level London ‘tube’
railways, the City & South London, carried its first
passengers in 1890. As far as signalling was concerned, the
application of electricity had been limited to the telegraphs,
early telephones, and some simple train detection devices.
impressed with the ‘Low Pressure Pneumatic’ system, and on
his return ordered a trial installation to be undertaken at
Grateley, between Andover and Salisbury, using air
compressed to only 15 pounds per square inch to operate
points and semaphore signals. In this case the ‘levers’ were in
the form of pull-out slides, with mechanical interlocking
between them, operating valves to control the compressed air.
Over the next few years, however, this was to change with the
coming of power signalling systems. These would take the
brute force out of signalling and change the focus of the
signalman’s job to being a regulator of traffic rather than a
manipulator of machinery.
A number of different power signalling systems emerged at
this time, each with its own disciples and claimed advantages.
The first to appear, in the closing months of the century
(January 1899) was the electro-pneumatic installation
(compressed air controlled by electrically-operated valves) at
Granary Junction, Bishopsgate (just outside Liverpool Street).
Figure 5: Low-Pressure Pneumatic frame at Grateley - 1901
It is interesting to note that Jacomb-Hood’s rationale for
adoption of the Low Pressure Pneumatic system was that, as
it involved no electrical devices, its installation, maintenance
and care could be carried out by skilled mechanical fitters
with no electrical knowledge, a good example of selection of
technology being made on the basis not only of suitability to
the application but also to the operational environment.
Figure 4: Interlocking Machine at Granary Junction - 1899
The interlocking ‘machine’, with its rotary handles in place of
levers, and parallel shafts reminiscent of a table football
game, was an American import and, as such, was viewed with
considerable suspicion by the conservative railway
establishment. It had none of the features that would come to
be associated with later power signalling installations such as
an illuminated track diagram, and even the mechanical
‘fouling bars’ that had long been fitted on the track to prevent
points being moved under a train were retained. But it showed
what could be done to take the hard manual labour out of
controlling a busy layout, and was followed immediately by
other, different, systems.
In 1900, the signal engineer of the London & South Western
Railway, J W Jacomb-Hood, visited America to inspect a
number of signalling installations. He was particularly
Yet another type of power signalling being introduced at this
time was the ‘All-Electric’ system, first brought into use at
Crewe, also in 1899. The system was designed and
manufactured by the London & North Western Railway at
their Crewe works, as had all their mechanical and electrical
signalling apparatus for over twenty years (the L&NWR had,
it seems, a deep and abiding distrust of contractors), and used
a miniature lever frame with two rows of levers spaced 4 - 4½
inches apart so as to allow the use of existing interlocking
components from full-sized frames.
These systems offered significant advantages in extending the
length of the signalman’s reach as operation of mechanical
points was limited by Board of Trade regulations to 200 yards
from a signal box (this was extended to 350 yards in 1925,
where it has remained ever since). Power operation, whether
electro-pneumatic, ‘low-pressure pneumatic’ or ‘all-electric’,
opened the way to a major extension of signal box control
areas, which the development of other devices would push
even further, and the early years of the 20th century saw some
extremely complex signalling installations brought into use.
The largest of these, commissioned at Glasgow Central in
1908, was controlled from a single frame of 374 miniature
levers, all mechanically interlocked.
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14 Emergence of Familiar Concepts
15 Train Detection
The early years of the 20th century were characterised by the
development, all over Britain, of what we would now call
‘urban transit systems’. First to come had been the
underground railways in London, followed by the deep-level
Tubes. Next on the scene were the new electric tramways,
providing fast (and clean) door-to-door transport at frequent
intervals. When in the early 1900s, the tramways started
pushing out into the suburbs, particularly in South London,
the railway companies were very concerned that tramways
were trespassing on their turf and started on a programme of
electrifying their suburban railway networks so that they
could compete with the tramways in an attempt to win back
their lost passengers.
Irrespective of how the points were moved (electric motors or
compressed air), or the signals shown to the drivers (arms or
lights), extension of a signalman’s area of control created one
particular and vital requirement: that of positively detecting
the presence of a train at a particular location and using the
information to protect it against other trains moving towards
it, or points moving underneath it.
In the beginning, the electrified services were operated using
traditional mechanical signalling methods of the type we have
already seen. However, as the electrified networks spread and
levels of train service increased, it became obvious that the
ability to operate a suburban network with frequent and fast
electric trains, together with longer-distance steam-hauled
services required some radical improvements in the signalling
arrangements. Not only was it necessary to increase line
capacity, but the ability to keep trains moving under all
weather conditions, particularly the dense fogs which London
and the other cities suffered, demanded an improved signal.
In November 1921 A E Tattersall read a paper to the
Institution of Railway Signal Engineers on ‘Three-Position
Signalling’, which turned out to be another defining moment
in the history of signalling. This paper examined the future
direction of signalling, what signals should be presented to
drivers, how these should be given, and how the system
should be controlled. As a result, the Institution set up the
‘Three Position Signal Committee’, which reported in
December 1924 that future signalling developments should
adopt three-aspect colour-light signals as their basis,
including the long-argued use of Yellow as the colour for
Caution signals. With this agreed, the way forward was
finally clear for signalling using the aspects we know today.
A further outcome of the IRSE’s Committee (now the ‘ThreeAspect Signal Committee’), in addition to the preference for
colour light rather than semaphore signals, had been the
recommendation that a fourth aspect be provided to allow
differentiation on lines carrying mixed traffic. In 1925, the
resignalling of London’s Blackfriars and Holborn Viaduct
stations (now part of the Thameslink route) introduced the
first four-aspect signal. Introduction of the Double Yellow, or
‘preliminary caution’ aspect, allowed not only more trains to
be accommodated on a given section of line, but for them to
run closer together. High-performance electric trains,
stopping frequently and rarely reaching more than 50 mph,
could drive confidently at this speed and not brake until
sighting the single yellow aspect, whilst heavier, faster and
less well-braked steam trains would start to brake on sighting
the double yellow aspect, giving them double the braking
distance.
The earliest practical means of achieving this, the Track
Circuit, had long since come of age, the original patent for
using an electric current flowing in the rails to detect the
presence of a train had been filed in the USA 50 years earlier,
in 1875. But its use had so far been limited to isolated
sections within manually signalled areas to remind the
signalman of a train waiting out of sight of the signal box; the
early power signalling installations such as Grateley and
Granary Junction had managed without them. However, from
the 1920s, the track circuit rapidly gained ground as the
essential safety feature of a signalling system, and as schemes
grew in size and complexity, track circuiting was always
there.
One of the track circuit’s principal benefits was its ability to
control signals (either power-worked semaphores or colourlights) without human intervention over long stretches of line.
Previously, with Absolute Block working, many hundreds of
small intermediate or ‘break-section’ signal boxes had been
provided to divide up stretches of line between stations and so
increase line capacity, allowing trains to run at more frequent
intervals, each of these having to be manned for two, or
sometimes three, shifts every day.
Now, these remote and often inaccessible boxes could be
replaced with one or more sections of ‘automatic signals’ so
that trains could safely follow one another at closer intervals,
each train protecting itself by replacing the automatic signals
to Danger as they passed, and allowing them to show
‘Proceed’ again when they had passed beyond the next signal
ahead. This method of operation, where signals are controlled
at and between signal boxes by track circuits rather than
signalmen communicating with block telegraph instruments,
is used up to the present time on all main lines and many
secondary lines in Britain, where it is known as ‘Track Circuit
Block’.
Where sections of automatic signals were provided so that
signal box control areas extended over several miles, there
was a need for the signalman to know what was going on
beyond his field of vision. The spread of power signalling
installations with continuous track circuiting was
accompanied by the appearance of the now familiar
illuminated track diagram. The first was installed in 1905 at
the station now called Acton Town as part of an electropneumatic signalling installation on the newly-electrified
District Railway, and their use eventually became universal.
13
From 1930 onwards, therefore, developments turned towards
signalling systems that could be controlled and interlocked by
entirely electrical means. The mechanical interlocking of the
miniature lever frames, and the watchmaker’s precision
required to install and set them up, would be progressively
replaced by electrical circuits, control panels, switches and
push-buttons.
Figure 6: Illuminated Diagram at Acton Town, District
Railway
Even away from the major station and junction areas, where
power signalling had not yet taken over from Absolute Block,
track circuits started to appear as aids to the signalman, for
example to prevent a lever being worked to move points
under a train, or to remind him of a train standing at a signal.
Track circuits could also be used to provide controls on block
instruments which, together with electrical proving of the
position of the signals, would impose the same discipline of
sequential block and signal operation on the signalman as
Sykes Lock & Block did mechanically.
16 The 1920s
Development of power signalling reached a peak in the 1920s
with the coming together of track circuits, colour-light
signals, power-operated points and miniature lever frames in
some of the most complex signalling schemes yet undertaken.
When the lines through London Bridge to Cannon Street and
Charing Cross were electrified this included total rebuilding
of the layout at Cannon Street and new signal boxes with
mechanically-interlocked miniature lever frames at Charing
Cross, Cannon Street, Borough Market Junction and London
Bridge. With a similar scheme in progress at the same time in
Manchester, this period marked the zenith of miniature lever
frame technology but also the beginning of the end of the era
of mechanical interlocking.
Operation and regulation of train services from the new
centralised power signal boxes at London Bridge and
Manchester Victoria was undoubtedly far more efficient than
it had been at the time when control of the layout was
dispersed between two, three or more signal boxes. However,
a down-side of the intricacy and compactness of the
interlocking mechanisms was that modifications (such as
when train lengths increased and there was a need to move
signals) were very complicated, difficult, time-consuming and
hence costly.
An interesting and very successful development at this time
was the electrically-interlocked miniature lever frame,
designed by the Westinghouse Brake and Saxby Signal
Company (now Invensys Rail), which followed the traditional
British practice of providing a lever for each signal and
individual set of points, but with the interlocking between the
levers achieved by means of contacts and electromagnetic
locks on the levers. To a signalman, it looked, felt, and
operated like any other lever frame, but the flexibility
provided by the electrical interconnections made this one of
the most popular methods of signalling control ever devised
and machines totalling nearly 10,000 levers, were built by
Westinghouse and exported all over the world. Although the
last new frame of this type was built in 1961, the ability to
rewire, reconfigure and recycle them meant that second-hand
frames of this type were still being installed up until the
1980s.
17 The Emergence of Route Setting
Mention of the ‘traditional British practice of providing a
lever for each signal and set of points’ is an appropriate way
of introducing the concept of Route Setting, which emerged
in Britain in the 1930s and has become the principle of
operation of all signalling systems developed since then,
including recent innovations such as computer-based
interlocking (CBI) and radio-based cab signalling.
Practice in Continental Europe, particularly in France, had
since the turn of the 20th century been to allow a signalman to
operate a single control device or ‘levier trajecteur’ to set up a
complete route through an interlocking area from the signal
that would authorise the train’s movement to its point of
destination ahead.
In 1922 the Great Western Railway (GWR) installed a trial
‘route setting’ signalling system at Winchester (Chesil)
station, on the now long-abandoned cross-country route from
Didcot to Newbury, Eastleigh and Southampton. The control
machine was a miniature frame, with 16 levers, and
movement of one lever through a number of intermediate
checking positions would move all the necessary points (if
they were free of interlocking or track circuit locking), prove
track circuits in the route Clear, and finally lower the signal,
an electrically-operated semaphore, for the train to proceed.
After four years of operation the GWR was sufficiently
impressed with this trial to go ahead with a major resignalling
scheme using route-setting lever frames at Newport East
(1927) and Newport West (1928). Although these
installations were revolutionary in themselves, the GWR did
not go so far as to abandon Absolute Block working between
14
the boxes. Nor did they pursue the route setting idea any
further, reverting to conventional miniature lever frames for
later major power signalling installations at London
Paddington, Bristol and Cardiff. Even the trial installation at
Winchester lasted only until 1933, when it was replaced by a
conventional (full-sized) mechanical lever frame.
Once embarked on this course, the L&NER vigorously
pursued the concept of the ‘relay interlocking’. Widening of
the East Coast Main Line north of York was going ahead,
funded by the Government as part of the scheme to relieve the
unemployment of the early 1930s, and as part of this work, a
new signal box was commissioned at Thirsk in 1935,
incorporating the first route setting panel on a British railway.
The method of route-setting used at Thirsk, and at many
major interlockings for the next thirty years, was that known
as ‘single control switch’, where operation of a single thumbswitch (or in some interlockings, a push-button) would set up
a complete route, providing the interlocking and controls
would allow it. In later years Westinghouse trademarked this
as the ‘One Control Switch’ or ‘OCS’ system, which reached
its peak of achievement with the massive installation
commissioned at York in 1951, controlling over 800
individual signalled routes and for some years the biggest
relay interlocking in the world.
A minor but nonetheless significant event that took place in
1938 should be mentioned at this point, for in that year a
small control panel and relay interlocking were commissioned
at Brunswick goods station in the Liverpool Docks. This used
an American technique of route setting, requiring the
signalman to operate first a rotary switch and then a pushbutton to define respectively the start point and the
destination of a signalled route. Its manufacturers, the General
Railway Signal Co of Rochester, New York, had coined the
name brand name ‘NX’ to describe this system - the
‘Entrance-Exit’ panel had arrived.
19 ‘Automatic Train Control’
Figure 7: ‘Route-Setting’ lever frame at Winchester - 1922
As a footnote to this part of the history of railway signalling,
the then new Ministry of Transport set up, in 1925, Britain’s
first electric road traffic signals at the junction of St James’s
Street and Piccadilly. Not only did this use three-aspect
colour-light railway signals, but they were controlled by a
police constable from a miniature lever frame, thereby
demonstrating the principle that technological advances are
best made by taking cautious steps forward and using
hardware that someone else has already developed.
18 ‘Levers versus Panels’
In the early 1930s, divisions were appearing between British
signal engineers as to how interlocking technology should
develop. On the one hand, the Southern Railway, with its
intensive suburban electric services, continued to install fouraspect colour-light signals worked from miniature, (but by
now, electrically-interlocked) lever frames. Elsewhere, the
London & North Eastern Railway (L&NER) was developing
the concept of a completely electrical system with no levers at
all and interlocking carried out in the control circuits by
relays, operated by switches on a control panel.
I mentioned earlier that for many years railway signalling
development tended to focus on protecting signalmen, rather
than drivers, from the consequences of their errors. From the
earliest days of railways and signalling, the fatal flaw in any
system using visual signals has been the possibility of the
driver overlooking, misreading or just ignoring the signal
giving him authority to proceed. Accidents without number
have occurred through drivers failing to see, or failing to
obey, signals at Danger, the situation that we nowadays call a
Signal Passed At Danger, or SPAD.
Attempts to solve the problem are almost as old as the
railways themselves. In 1840 the London & Birmingham
Railway experimented with a means of giving audible and
visual signals to a driver on a locomotive if he passed a signal
at Danger by means of a lever on the track which engaged a
lever below the locomotive to sound a whistle and turn a red
lamp in the driver’s face. Adoption of such a device,
however, at a time when railway companies were arguing
against even the use of block telegraphs and other aids to
safety on the basis that they would make signalmen and
drivers careless, would not happen for many years.
In 1900 the GWR, prompted by a fatal SPAD accident at
Slough, started experiments with an electro-mechanical
system of ‘Automatic Train Control’ (ATC) which sounded a
15
horn in the locomotive cab to warn the driver on the approach
to a signal showing Caution, or a bell to confirm that it was at
Clear. If the driver did not acknowledge the horn signal
within 2 – 3 seconds, the train’s brakes (continuous vacuum
brakes on the GWR) would be applied automatically and
bring the train to a stop.
The system, which was activated by contact between a ramp
mounted on the track and a ‘shoe’ on the locomotive, was
subsequently installed on the GWR’s main line from
Paddington to Reading, and eventually (by the early 1930s)
all over the GWR network, from which it finally disappeared
only in the 1970s, well into the British Railways (BR) era.
Until nationalisation in 1948, railways in Britain had been
private, commercially-driven bodies, each with its own
operational and engineering practices, preferences and
prejudices, as illustrated by the proliferation of designs for
lever frames, signals, and block telegraphs discussed earlier.
Even when 120-plus railway undertakings were combined
into the ‘Big Four’ companies (Great Western, London
Midland & Scottish, London & North Eastern and Southern)
in 1923, many of the old ways remained. For a railway to
adopt a system developed by another company was
unthinkable and away from the GWR, widespread adoption of
ATC or any other protection or warning system did not occur
until well after nationalisation. During the intervening years,
accidents caused by drivers ignoring or misreading signals
consequently continued to occur, and the Railway
Inspectorate continued to argue in vain for adoption of ATC.
After the formation of BR in 1948, one of the first actions of
the new British Transport Commission was to undertake the
installation of ATC throughout the principal rail routes of its
network. Trials were started, using a system of track-mounted
magnets to operate a horn or a bell in the driver’s cab, similar
to the GWR ATC, but with the addition of a visual indicator
to remind the driver that he had received a warning, and that
it had been acknowledged. Soon after the new system went on
trial on the East Coast Main Line in 1952, a catastrophic
double collision occurred at Harrow in which 112 passengers
and train crew were killed, and the Inspecting Officer had no
hesitation in stating once again that ATC would have, without
a doubt, prevented this accident.
Figure 8: Track-mounted magnets of BR-AWS system
By the time trials of the new system had been completed,
reports prepared and approval given, work had only just
started before another horrific rear-end collision following a
SPAD, in fog, at St Johns in south-east London. Installation
of the Automatic Warning System or ‘AWS’ as it came to be
called, went ahead steadily and gradually extended to all BR’s
main lines and most of the secondary lines, where it remains
in use.
As an interesting contrast to the timescales for development
of ATC in Britain, railways in France were being equipped
with a warning system similar to the GWR ramp as early as
1875. Called the ‘Crocodile’, it acted in a similar way to ATC
on the approach to a signal (that is, as a vigilance device), and
required the driver to acknowledge a warning within 4 – 5
seconds, or the brakes would be applied. It has been
suggested that GWR’s ATC was based on the Crocodile
concept but, whereas ATC has now passed into history, the
wavy ramp of the Crocodile remains a familiar feature on
main lines throughout France, Belgium and Luxembourg,
over 130 years since its introduction.
20 Automatic Train Protection
AWS is a ‘warning system’, and is required to work with
trains of widely differing weights, speeds and performance
characteristics. For this reason it does not have the ability to
give the unequivocal Stop command of the ‘trainstop’ devices
used on London’s Underground and other metro systems.
Also, because of the driver’s ability to acknowledge a
warning and prevent the brake application, it will not provide
protection in cases where signals are deliberately ignored or
misunderstood. The only system that can provide this level of
protection, as well as constant supervision of speed, is
Automatic Train Protection or ATP.
Following several serious SPAD accidents in the late 1980s
the Government made an undertaking that BR would fit ATP
throughout its network within five years (this somewhat rash
promise was later modified to ten years). Because no British
signalling manufacturer could at that time deliver an off-theshelf technical solution, BR initiated two trial ATP
installations. The two main routes between Paddington and
Bristol were equipped with a system called ‘TBL’ from
Alstom in Belgium, and the German ‘Selcab’ supplied by
Alcatel (now Thales) was installed on the Chiltern Line routes
between Marylebone and Banbury. Both systems used a
system of track mounted loops and transponders to relay
information from the trackside to the train regarding the state
of the signals and the permitted speed on the line ahead so
that the driver could control the train within an envelope of
protection, under the supervision of the ATP.
Implementation of both trial systems was well under way by
1994, when the newly-formed Railtrack announced that a
financial case for system-wide installation of ATP could not
be made and so would not proceed further. The trial
installations were nevertheless completed, and will continue
in service until full transmission-based signalling designed to
European interoperability standards is installed in the future.
16
When BR initiated its ATP trial projects in 1991 the Channel
Tunnel was still under construction, and realisation of the
‘European Rail Traffic Management System’ concept
(ERTMS) to allow operation of trains with a common
signalling system throughout the member states of the
European Union was some years away.
Having already recognised that it would be many years before
ERTMS would be implemented throughout the British
railway network, Railtrack had started investigating an
interim train protection solution. The Train Protection &
Warning System (so called because its trainborne equipment
combines the function of a trainstop with that of the existing
AWS) was based on available technology and effectively
offered a ‘low-tech’ solution to mitigate the consequences of
SPADs. Trials of the equipment were put in hand in 1996 on
the Thameslink network, and the Thameslink (now First
Capital Connect) electric multiple unit (EMU) trains were
fitted with the train-carried equipment.
During the time that this work was going on, two major
SPAD accidents occurred, firstly at Southall in September
1997, and then at Ladbroke Grove in October 1999. The latter
accident occurred less than two months after Parliament had
enacted the Railway Safety Regulations 1999 in which the
provision of a system of train protection on all trains, and at
‘selected signals’ (those protecting junctions and points of
conflict) was mandated by law, to be completed by the end of
2003. At the time of writing (2010) TPWS is installed at
12,000 ‘selected signals’ throughout the British main line
network and on all trains, where it will remain in use, together
with AWS, for the foreseeable future.
21 Towards the Present Day
We left the railway signalling scene at the outbreak of war in
1939. For obvious reasons, any advances in signal
engineering were effectively stopped for the duration of the
war, the railways’ first priority being to just keep going.
Major schemes such as the resignalling of York and
electrification from Liverpool Street to Shenfield were put on
hold, and others were abandoned, never to be resurrected.
Meanwhile, the railway system struggled on in the face of
shortages of material, depletion of its manpower, and enemy
attack.
way forward (except on the Southern Region, successors to
the Southern Railway, who were still busily installing
miniature lever frames in the South London suburban area),
and implementation of AWS got under way, as we have seen,
although funds for new major schemes were limited until the
BTC’s Plan for the Modernisation and Re-equipment of
British Railways was announced in 1955.
The Modernisation Plan included over £100 million to be
expended on signalling schemes, which would accelerate the
replacement of semaphore signalling with colour-lights and
track circuiting, to allow faster train services. It would
include, among a myriad of other schemes, electrification and
resignalling of the West Coast Main Line from London to
Crewe, Liverpool and Manchester and further extension of
the Southern Region’s electrified lines. Also, within ten years,
steam locomotives would disappear from British Railways, to
be replaced by either diesel or electric traction (BR’s last
steam-hauled train actually ran in August 1968).
An important technological advance around 1960 was the
appearance of electronics, initially used for the remote control
of signalling interlockings over distances beyond which
individual signalling cables, with a separate pair of wires for
each individual function, became prohibitively expensive.
Systems of this type based on telephone exchange technology
had been in use for many years in CTC (Centralised Traffic
Control) schemes in the British Commonwealth and the USA,
but their slow speed of operation and response made them
unsuitable for main line railway use.
What was required was a method of exchanging controls and
indications between a controlling signal box and a remote site
instantly, or at least within no more than a second, over a
single pair of wires. The first such system, using transistors,
was brought into use on the Crewe to Manchester line in
1959, where a number of ‘satellite’ relay interlockings were
supervised by electronic remote control from new signal
boxes at Sandbach and Wilmslow. Once such systems had
become available and reliable, it would be possible to extend
control areas almost without limit so that the area under the
supervision of a single signal box became more an issue of
operational preference than technology.
After 1945, nothing really changed for the first year or so but
then mothballed schemes were started up again and
completed. But with the companies exhausted from six years
of war and misuse, the whole system was run-down and in
need of serious repair and investment, which would come
only as a result of drastic change. On Nationalisation Day, 1st
January 1948, custody of the Big Four railway companies
passed to the British Transport Commission (BTC) and
British Railways was born.
On the signalling front, the pattern of British railway
signalling practice and principles was pretty well established
by the 1950s. New signalling schemes using relay
interlockings and control panel operation were accepted as the
Figure 9: Glass-enclosed (‘fish tank’) signalling relays
17
The other principal technical advance at this time (broadly
1955 to 1965) was that of Miniaturisation. In 1958 the IRSE
had set up a Miniaturisation Committee to consider the
requirements and characteristics of signalling relays. In the
previous 20 years, signalling relay design had progressed
from large, individually wired shelf-mounted relays (‘fish
tanks’) to plug-in units where exchange of a relay required no
wires to be disconnected. Size was still a problem, however,
and with the advent of the new electronic technologies, the
size of relays and the room needed to accommodate them was
becoming a serious consideration.
The IRSE Committee reported in 1963 and recommended
adoption of a standard range of what are now called
‘miniature signalling relays’. IRSE miniature relays (or as
they became known from the series of BR Specifications
detailing their construction and functionality, ‘BR 930
relays’), became one of the most widely used signalling relays
of all time so that, even in these days of electronics, they are
still in use in their tens of thousands.
SSI, although remarkably ‘low-tech’ by comparison with
current computer technology, has been an outstanding
success. Developed under a three-way agreement between
BR, Westinghouse (now Invensys Rail) and GEC-General
Signal (now Alstom), it is the nearest thing there has ever
been to a standardised British signalling interlocking.
Numerous SSI installations have been commissioned
throughout the BR network since 1985 and it has been
successfully sold to a number of overseas railways. SSI
interlockings can be controlled from either ‘Entrance-Exit’
control panels, or from screen-based workstations on which a
mouse or trackerball is used to point-and-click on symbols
representing the physical buttons or switches.
Alongside the miniaturisation of relays, signalling suppliers
devoted considerable ingenuity to reducing the size of control
panel components so as to allow ever-expanding signal box
control areas to be represented on control panels of a practical
size. Manufacturers produced ‘modular’ or ‘mosaic’ control
panels in which standardised components were assembled on
to square or rectangular tiles which then plugged into a baseframe structure.
22 New Interlockings
We have already seen something of the origins of relay
interlocking and control panels, and the 1955 BR
Modernisation Plan envisaged a huge expansion of colourlight signalling and control panel signal-boxes. Faced with a
shortage of signalling interlocking circuit designers,
manufacturers developed ‘Geographical’ interlocking systems
in which pre-wired and machine-tested packages of relays
controlling individual signals and points are connected
together by multi-core cables to provide the interlocking
functions required for any track layout.
The last 30 years have seen an explosion of electronics into
all walks of life. Railway signal engineers, by nature
conservative folk, have been slow to adopt electronic
technology for safety-critical systems but communications,
information systems, remote control and ‘human-machine
interfaces’ (i.e. control panels) have all incorporated
electronics to great advantage.
The most dramatic advance on Britain’s railways in the recent
past has been the adoption of processor-based interlocking
systems. These had been used in Continental Europe for many
years, but in 1985 there were still none on the BR network. In
September of that year, however, the first Solid State
Interlocking (SSI) was commissioned at Leamington Spa and
Britain’s railways entered the age of the safety-critical
processor.
Figure 10: Workstation at a BR Integrated Electronic Control
Centre (IECC)
In the eyes of many signal engineers, SSI is still the system of
choice. Attempts to introduce other processor-based systems
on to the British railway network from Continental Europe
have so far met with only limited success, their suppliers
having encountered problems not only with assurance and
acceptance of the equipment, but also with the configuration
changes required to adapt non-British systems to British
signalling principles and operating practices.
Another staple component of Continental signalling practice,
the axle counter method of train detection, is now being
widely adopted by Network Rail. Unlike track circuiting,
which uses an electric current in the rails to continuously
detect the presence of a train’s wheels and axles on a section
of line, an axle counter works by indicating a section as
Occupied when it detects and counts the passage of a train’s
wheels into the section. It sets the section to ‘Clear’ again
only when it detects the same number of wheels leaving, in
much the same way as signalmen operating Absolute Block
observe and record the entry of a train into a section, and its
subsequent departure.
Which brings us neatly back to the ‘Bobby’.
18
23 The Future
Acknowledgements
In an earlier section I mentioned the European Rail Traffic
Management System or ERTMS. In April 2006 the Railways
(Interoperability) Regulations came into force, mandating
implementation of a standardised signalling and train control
system in line with the EU Interoperability Directive. A brief
consideration of Eurostar will show why this is a sound idea –
at present a train travelling from St Pancras to Brussels
requires three different signalling systems, two for France and
one for Belgium (until the dedicated High Speed Link to St
Pancras was opened in 2007 this included AWS and TPWS
for running over Network Rail’s lines into Waterloo).
The author would like to thank the many friends and
colleagues from main line, metro and heritage railways who
have knowingly or unknowingly contributed to this paper
during his 40 years of working in signalling, and to Lloyd’s
Register Rail Ltd for this opportunity to present what I hope
you will find a stimulating introduction to the subject.
I would also like to acknowledge the help of the Institution of
Railway Signal Engineers, from several of whose publications
I have extracted facts, figures and photographs, in particular
Figures 4, 5, 6, 7 and 10.
These Regulations do not require retrospective compliance, so
installation of ERTMS on the High Speed rail routes defined
in the legislation will take place only when the existing
signalling systems are replaced. In the meantime, however, a
trial installation has been placed into service on the Cambrian
Coast line in North Wales, between Shrewsbury, Aberystwyth
and Pwllheli. The term ‘ERTMS’ is a collective name for the
combination of the ‘European Train Control System’ or
ETCS (which provides the ‘interlocking’ part of the
signalling) with GSM-R, the standardised mobile radio
communication system for railways.
The system installed on the Cambrian Coast is ERTMS Level
2, which provides continuous communication between the
interlocking (located at the Control Centre) and the train via
GSM-R radio. Additional communication between fixed
points on the track and the train provides position references
and direction information by means of groups of transponders
or ‘balises’.
Train detection is by means of axle counters, and trains
additionally report their position back to the control centre,
based on the distance travelled since passing the last balise.
There are no lineside signals, permission for the driver to
proceed (the ‘Movement Authority’) being transmitted by
radio, based on the train’s own position, that of the train or
trains ahead, the correct setting of points and the permitted
speed. Today’s driver of an ERTMS-equipped train on the
Cambrian Coast line can therefore watch speed, distance and
movement authority being displayed on a screen in the cab
and control the train confident in the envelope of protection
that the signalling system is providing, which will brake the
train to a stop if any of these are exceeded.
At the time of writing, plans are being developed to install
ERTMS Level 2 signalling throughout the Great Western
Main Line from London Paddington to Bristol and the West
of England, the supervision of which will be concentrated in
one Control Centre. So ends, for the time being, the 180-year
journey that started on a wet September day on the Liverpool
and Manchester Railway, where the driver crosses his fingers
and peers through the gloom and the pouring rain to catch a
glimpse of the bobby’s hand signal.
19
Preamble and Forethoughts
A History of Railway Signalling
‘from the Bobby to the Balise’
Railway Signalling and Control is a complex and absorbing subject, with a history
stretching back to the dawn of the Railway Age 180 years ago. This presentation,
and the accompanying paper in the Course Book, is a personal view of that history,
as Britain’s railways and their technologies have developed since 1830.
Signalling history can be divided into three periods of (very approximately) 60 years:
• from 1830 to 1890 – evolution and adoption of the basic requirements for
safe operation.
• from 1890 until after the Second World War – development of new electrical
signalling technology and principles.
• from the 1950s to the present – electronics, computers and the evolution of
new methods of signalling.
Stephen Clark
Principal Consultant
21 May 2012
This presentation is therefore a gallop, or at least a quick trot, through that history.
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
What is Railway Signalling all about?
What is Railway Signalling all about?
For the 21st century railway, signalling means:
For the 21st century railway, signalling means:
• A system of protection for rail traffic, based on:
• Safe separation of moving trains.
• Prevention of conflicts between trains at junctions.
• Prevention of conflicts between trains and infrastructure.
• A system of protection for rail traffic, based on:
• Safe separation of moving trains.
• Prevention of conflicts between trains at junctions.
• Prevention of conflicts between trains and infrastructure.
• A system of control to provide information to drivers by means of:
• lineside signals and signs.
• route indicators.
• in-cab information.
• A system of control to provide information to drivers by means of:
• lineside signals and signs.
• route indicators.
• in-cab information.
• A system of supervision to enable operators to:
• direct and route trains to their destinations.
• re-route and divert trains to mitigate the effects of disruptions.
• deliver train services in accordance with a published timetable.
• A system of supervision to enable operators to:
• direct and route trains to their destinations.
• re-route and divert trains to mitigate the effects of disruptions.
• deliver train services in accordance with a published timetable.
• A system of management to:
• allow maximum use of the railway’s capacity.
• A system of management to:
• allow maximum use of the railway’s capacity.
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
The Origins of Signalling – Keeping Trains Apart
The Origins of Signalling – Keeping Trains Apart
For those of us used to driving cars, we know how far it takes to stop in an emergency.
But for the driver of an early steam locomotive, stopping in an emergency was an
entirely different situation.
The Highway Code tells us, for a typical car, with an alert driver, on a dry road . . .
At 50 mph . . .
if an obstruction is
sighted here . . .
15m (reaction distance)
your car stops here
38m (braking distance)
53m
This shows typical braking performance for a modern car with efficient and wellmaintained brakes and tyres in good condition – in railway terminology, we can ‘drive
on sight’.
And remember - the action of braking, once you have reacted, requires you only to
move your foot from the accelerator pedal to the brake.
Not only are the laws of physics different . . .
• the coefficient of friction between a metal wheel and a metal rail (typically 0.3 or
less) is much lower than a rubber tyre on an asphalt road.
. . . but the physical act of braking an early steam-hauled train required the driver to:
• whistle to tell the guard at the back of the train that brakes were required.
• close the regulator.
• apply the locomotive brakes (usually a hand-wheel and screw).
• apply sand to the rails to increase adhesion.
• often, reverse the valve gear, and
• open the regulator to apply still more braking effort . . .
• . . . and keep his fingers crossed.
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
20
The Origins of Signalling – Keeping Trains Apart
The Origins of Signalling – Keeping Trains Apart
Consequently, when the driver has done everything required to ‘bring his train under
control’, the stopping distances look very different to those for a car . . .
At 50 mph (22m/s) . . .
if an obstruction is
sighted here . . .
>100m (5s) for reaction
and brake application)
the train stops here
1000 - 1500m braking distance
up to
1500m
For a train that takes nearly a mile to stop, ‘driving on sight’ is clearly not an option.
And - although train braking is the concern of the rolling stock engineer, not the signal
engineer, no signalling system can ensure safe operation of the railway unless trains
have effective brakes.
The railway is divided into a number of ‘Block Sections’ and only one train is permitted
to enter a Block Section at a time.
In the 1830s, the only
practical way to enforce the
Block system was by means
of a minimum time interval.
The system was operated by
the railway’s own policemen
(‘Bobbies’) at the lineside,
giving hand signals to trains
and timing them with a sandglass.
But Time Interval working could not give an ‘absolute’ assurance that a block section
was unoccupied before a following train was allowed to enter it.
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
The Origins of Signalling – Keeping Trains Apart
The Origins of Signalling – Keeping Trains Apart
From the 1850s, Electric Telegraphs
allowed messages to be exchanged
between signalmen so they would
know when it was safe to allow a train
to proceed.
The Three-Position Block Telegraph
Instrument shown here simplified
operation by providing for one of
three standard messages to be sent:
• ‘Line Clear’
• ‘Train On Line’
• ‘Line Blocked’ (the normal
state, with no trains in or
approaching the section)
Early telegraph instruments were slow
and cumbersome to use. Messages
had to be spelt out, letter by letter.
These instruments provide the basis of a
safe method of signalling that is in use to
this day.
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
The Origins of Signalling – Keeping Trains Apart
The Origins of Signalling – Keeping Trains Apart
From the 1840s, fixed signals started to
be used, instead of the Bobbies’ hand
signals, to convey instructions to train
drivers.
A word about Accidents . . .
We are used today to the idea of ‘engineering for safety’.
But with no experience on which to draw, evolution of safety was slow and
reactive. Accidents were the major incentive for change.
These took a number of different forms,
ranging from the Great Western Railway’s
rotating Disc and Crossbar signal . . .
Anyone interested in the history of signalling should read ‘Red For Danger’ by L. T.
C. Rolt, first published in 1955 but updated since.
. . . to various types of semaphore signals
with moveable arms.
As with the Block Telegraph instruments,
many signals of this type remain in use.
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
21
The Origins of Signalling – Keeping Trains Apart
The Origins of Signalling – Keeping Trains Apart
After 1860, the next significant step in the evolution of British signalling was the
development of Interlocking.
Interlocking is required to ensure that a signalman operates points and signals such
that unsafe train movements are prevented, for example:
• signals controlling movements that conflict at a set of points
• signals controlling movements that directly ‘oppose’ other movements (i.e. that
lead to a head-on collision)
• showing a signal allowing a driver to proceed with points incorrectly set.
Interlocking of points and signals is achieved by interconnecting the levers controlling
them through a mechanism that will allow only certain combinations of levers to be
operated, or to ensure that operation takes place in a safe sequence.
A History of Railway Signalling
A History of Railway Signalling
LLOYD’S REGISTER RAIL
LLOYD’S REGISTER RAIL
The Origins of Signalling – Keeping Trains Apart
Signalling - the March of Technology
This picture shows a typical Lever Frame
as it would have been supplied from the
manufacturer’s works.
During the last quarter of the 19th century there was a flourishing of technical invention
and innovation, including:
• train detection systems – track circuits and treadles to assist safe operation;
• ‘enhanced’ block systems – interlinking of signal levers with the block
instruments to enforce safe and sequential operation;
• improved control of single-line working;
Floor
level
and, in the last few months of 1899:
• Britain’s first installation of power-operated signalling
Interlocking
mechanism
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
Signalling - the March of Technology
Signalling - the March of Technology
Power signalling opened the way for colour light signals to start
replacing mechanical semaphores.
With power operation, the signalman was
operating only valves or switches to move
points and signals, so the levers controlling
them could be miniaturised.
Use of colour-light signals,
together with continuous
track circuiting to detect the
presence of trains, allowed
long stretches of railway to
be controlled
automatically . . .
Glasgow Central signalbox (installed in 1907, replaced in
1960) with 374 mechanically-interlocked miniature levers.
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
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Maximising line capacity
Signalling - the March of Technology
Power signalling opened the way for colour light signals to
start replacing mechanical semaphores.
Coulsdon North
Cane Hill
Quarry
Worstead Green
Earlswood
Earlswood Station
Salfords
Horley North
Horley South
Gatwick
Tinsley Green
Three Bridges North
Three Bridges
Three Bridges South
Balcombe Tunnel
Balcombe Intermediate
Balcombe Station
Stone Hall
Copyhold Jn
Haywards Heath North
Haywards Heath
Folly Hill
Wivelsfield
The 36 signal boxes previously required
Keymer Jn
Burgess Hill
Lovers Walk
for normal operation were reduced to
Hassocks
Brighton North
only 8, located at the junctions.
Clayton Cutting
Brighton South
A further 9 signal boxes could be ‘opened
Brapool Cutting
Brighton West
Patcham
when required’, but normally their signals
New England
Preston Park
Holland Road
would work automatically.
from
London
Use of colour-light signals,
together with continuous
track circuiting to detect the
presence of trains, allowed
long stretches of railway to
be controlled
automatically . . .
The use of colour-light signals spaced at regular intervals, worked automatically
wherever possible, allowed maximum use to be made of a line’s capacity without the
need for large numbers of individual signalboxes and their associated ‘bobbies’.
c. 1km
c. 1km
c. 1km
c. 1km
Train 1
Resignalling from Coulsdon to
Brighton – 1932
covering 36 route miles
BRIGHTON
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
Maximising line capacity
Maximising line capacity
The use of colour-light signals spaced at regular intervals, worked automatically
wherever possible, allowed maximum use to be made of a line’s capacity without the
need for large numbers of individual signalboxes and their associated ‘bobbies’.
The use of colour-light signals spaced at regular intervals, worked automatically
wherever possible, allowed maximum use to be made of a line’s capacity without the
need for large numbers of individual signalboxes and their associated ‘bobbies’.
c. 1km
c. 1km
c. 1km
c. 1km
c. 1km
c. 1km
c. 1km
c. 1km
Train 1
Train 1
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
Maximising line capacity
Signalling – maximising line capacity
The use of colour-light signals spaced at regular intervals, worked automatically
wherever possible, allowed maximum use to be made of a line’s capacity without the
need for large numbers of individual signalboxes and their associated ‘bobbies’.
The use of colour-light signals spaced at regular intervals, worked automatically
wherever possible, allowed maximum use to be made of a line’s capacity without the
need for large numbers of individual signalboxes and their associated ‘bobbies’.
c. 1km
c. 1km
c. 1km
c. 1km
Train 1
c. 1km
Train 2
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
c. 1km
c. 1km
c. 1km
Train 1
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Modern developments
Modern developments
The use of colour-light signals spaced at regular intervals, worked automatically
wherever possible, allowed maximum use to be made of a line’s capacity without the
need for large numbers of individual signalboxes and their associated ‘bobbies’.
. . . from miniature levers to control panels in the 1950s . . .
c. 1km
c. 1km
Train 2
c. 1km
c. 1km
Train 1
This method of operation, which is known as ‘Track Circuit Block’, using multipleaspect colour-light signals controlled by a system of train detection, remains the
standard signalling architecture on British main lines.
In the meantime, however, the technology of the signalbox itself has continued to
evolve . . .
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
Modern developments
Modern developments
. . . to very large control panels in ‘Area Signalling Centres’ in the 1970s and 1980s. . .
. . . to VDU screens and computer-based interlockings in the late 1980s and 1990s.
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
Other signalling developments
Other signalling developments
In addition to the development of signals, interlockings and control systems, the last
third of the 180-year history of signalling has seen the appearance of a number of
important technical improvements.
The most recent – and significant – development is the successful commissioning of a
trial ‘European Railway Traffic Management System’ (ERTMS) on the Cambrian Coast
line from Shrewsbury to Aberystwyth and Pwllheli.
• Automatic Warning System (AWS) – to remind a driver that the next signal
ahead is not showing a ‘Proceed’ (Green) aspect and that action is required to
prepare to stop.
With ERTMS Level 2 a Movement Authority (MA) for a train to proceed is transmitted from
the Interlocking to the train by radio and displayed in the cab – the driver then controls the
train in accordance with this authority (permitted speed, distance to go).
• Train Protection & Warning System (TPWS) – to bring a train to a stand if it
passes, or appears likely to pass, a signal showing a Red (Danger) aspect.
• If the driver exceeds the MA,
the system will intervene and
the train will be braked.
• Axle counters – train detection
irrespective of track condition, adverse
weather, or railhead contamination.
• No lineside signals are
required, only fixed markers.
• Train detection is by means
of axle counters, connected
to the Interlocking.
• Position references are given by
‘balises’ (transponders)
mounted on the track.
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
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Future signalling developments
A History of Railway Signalling
What next . . .
That may well be up to some of you here today!
Thank you for listening!
A History of Railway Signalling
A History of Railway Signalling
LLOYD’S REGISTER RAIL
LLOYD’S REGISTER RAIL
A History of Railway Signalling
Acknowledgments:
• To the Institution of Railway Signal Engineers, for inspiration, facilities and photographs
• To colleagues in Lloyd’s Register Rail, for support and encouragement
For more information, please contact:
• To Siemens Transportation, Invensys Rail Ltd and Mark Glover, for photographs
• To Jim Dapré, for the photograph on the title slide
Lloyd’s Register Rail Ltd
71 Fenchurch Street
London, EC3M 4BS
T +44 (0)20 7423 2943
F +44 (0)20 7600 1441
E stephen.clark@lr.org
A History of Railway Signalling
A History of Railway Signalling
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LLOYD’S REGISTER RAIL
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