A Pressured Tank Car Inspection System for Railroad
Transportation Security
P. C. Womble*, J. Spadaro**, M. A. Harrison**, A. Barzilov*, D. Harper*, B. Lemoff**, R. Martin**, I. Novikov*, J. Paschal*,
L. Hopper*, C. Davenport*, and J. Lodmell*
*
Applied Physics Institute, Western Kentucky University, 1906 College Heights Blvd MS 11077, Bowling Green, KY,
42101-1077, phone: 270 781 3859email: womble@wku.edu,
**
Institute for Scientific Research, Inc., 1000 Galliher Dr, Fairmont, WV 26555-2720
Abstract--Pressurized rail tank cars transport large
volumes of volatile liquids and gases throughout the
country, much of which is hazardous and/or flammable.
Our group is developing a trackside inspection system for
these tank cars. It consists of five narrow frequency band
pressure sensors with center frequencies of 40 and 75
kHz, a broad band microphone for sound normalization
and three video cameras. In addition, a 5 cm x 5 cm
NaI(Tl) radiation detector provides radiological data on
the passing trains every 60 seconds. During operation,
an audio frequency spectrum is associated with each
frame of the video camera as the train passes by the
system at normal speeds and the spectra are inspected for
high frequency sounds associated with leaks. A 10 m tall
tower houses the system position approximately 10 m
from the center of a rail line and siding located in
Bowling Green, KY. The system is controlled by a
website and server located at the tower and the Internet
connection utilizes WiFi (802.11g) radios.
1. INTRODUCTION
Annually, approximately 1.7 to 1.8 million carloads of
hazardous materials shipped by rail [1], 2/3 of which are
transported by tank car [1]. In 2005, The US Department
of Transportation recorded 737 incidents of unintentional
release of hazardous materials from rail cars, of which 48
incidents were due to train derailment [2]. Of the total
incidents, 79 were identified as serious incidents 1, causing
fatalities, serious injuries, major evacuations, or
substantial release of hazardous materials. The greatest
concern lies in unintentional release of toxic-by-inhalation
(TIH) gases, such as chlorine and anhydrous ammonia,
which predominantly move by rail and recently have been
among the top 10 rail-shipped hazardous materials by
volume [3].
1
PHMSA revised definition (2002) defines a major incident as one that:
causes a fatality or major injury due to the release of a hazardous
material, causes the evaluation of 25 or more personans as a result of
release of a hazardous material or exposure to fire, causes a release or
exposure to fire that results in the closure of a major transportation
artery, results in the alteration of an aircraft flight plan or operation,
involves the release of radioactive materials from Type B packaging, is a
release of over 11.9 gallons or 88.2 pounds of a severe marine pollutant,
or involves the release of over 119 gallons or 882 pounds of a hazardous
material.
Despite this risk, railroads are federally-mandated to
transport TIH gases through the railroads’ common
carrier obligation, a situation which causes grave concerns
for both the railroad industry and the communities
through which commodities travel:


According to Edward R. Hamberger, President
and CEO of the Association of American
Railroads, railroads face a potentially ruinous
liability with each TIH shipment [1]. Though
such shipments constitute only 0.3% of all rail
carloads, TIH contributes about 50% to the
overall cost of railroad insurance. Even with this
substantial expenditure, it is not possible to be
fully insured against a catastrophic incident, and
railroads have suffered multi-billion dollar
judgments, even when not at fault.
The District of Columbia city counsel recently
voted to prohibit the transport of hazardous
materials by rail through Washington DC, due to
unacceptably high risk of inhalation hazards.
The law was overturned on appeal, because
rerouting merely shifts hazardous materials risk
to other communities and to rail track lines less
able to handle such shipments [4]. Nevertheless,
similar efforts are under discussion in Atlanta,
Baltimore, Boston, Cleveland, Chicago, Las
Vegas, Philadelphia, Pittsburgh, as well as all of
California1.
Of the solutions proposed to mitigate the risk of TIH
release incidents, which include rerouting, shipment
notification, and wayside monitoring, only the latter is
supported by the railroad industry as a practical solution.
However, current wayside monitoring systems do not
address tank car integrity or even TIH release detection;
instead, they focus on reducing risk of train derailment by
monitoring wheel track defects known to cause
derailments.
Furthermore, the current wayside
monitoring system does not specifically target urban areas
or address their concerns not only of accidental release,
but also of terrorism.
Worldwide, approximately 181 terrorist attacks on trains
and related rail targets occurred between 1998 and 2003,
resulting in approximately 431 deaths and thousands of
injuries[5]. The March 11, 2004 train bombings in

Figure 1. A tank car leaks hydrochloric acid in
Fulton, KY on October 19, 2006. (Photo courtesy: H.
Gantt and M. Garland, Murray State University)
Madrid, Spain involved 13 IED devices, resulting in 10
near-simultaneous explosions that killed 191 people and
injured over 1700. A similar scenario could be used for
an attack on a rail tank car, or an attack could easily occur
at a distance and while the train was in motion. For
instance, 50 caliber sniper rifles, weapons readily
available in the US and known to be possessed by alQaeda, are known to have the capability to puncture
railroad tank cars at distances of 1000 to 2000 yards[6].
To reduce the anxieties of urban rail hubs and to mitigate
the effects of both unintentional hazardous materials
(HAZMAT) releases and those deliberately caused by
terrorism, there is a need to augment waypoint detection
systems to include:




Early detection of HAZMAT release events,
particularly focusing on TIH or explosive gases.
These sensors could include chemical detection
sensors, especially for the most dangerous
substances transported through a given area, and
more general acoustic leak detectors to detect
general releases from rail tank cars, regardless of
substance.
Reporting to railroad companies and community
HAZMAT first response systems, in order to
facilitate early response to hazmat release events
Placement of these waypoint detectors after rail
yards, in particular to check that no leak
detection events occurred during “storage-intransit”, a time in which tank cars may be
unmonitored and more at risk for terrorist
attack3.
Placement of HAZMAT waypoint detectors
outside urban areas, for detection of leak events
prior to the entrance of the train to an urban area.
Additional waypoint detectors placed throughout
track within an urban center may also facilitate
early warning of release events.
Inclusion
of
radiological
detectors:
approximately 2 million pounds of spent nuclear
fuel were transported by rail in the US between
1979 and 1996 [3], an amount expected to
substantially increase when the Yucca Mountain
facility begins accepting waste.
While a
National Academy of Sciences panel on the
safety and security of hazardous material rail
shipments concluded that the risks of transport of
spent nuclear fuel were low compared to that of
other hazardous materials, they also concluded
that that public anxiety and perception makes the
shipment of radioactive material a significant
issue3.
We have developed a trackside inspection system for
these tank cars. It consists of five narrow frequency band
pressure sensors with center frequencies of 40 and 75 kHz
and a broad band microphone for sound normalization.
For visual inspection purposes, three cameras are
connected to a digital video recorder. In addition, a 5 cm
x 5 cm NaI(Tl) radiation detector provides radiological
data on the passing trains. In the following paper, we will
discuss our design and its capabilities.
2. DETECTION OF TANK CAR LEAKS
High pressure gas leaks are known to create broadband
acoustic emission in the audio and ultrasound regime.
Turbulence created as the high velocity gas stream enters
the surrounding air leads to periodic vortex shedding
which in turn leads to the creation of acoustic waves. The
acoustic emission is dependent on the velocity of the gas,
the leak size and shape, as well as the density of the gas
and surrounding air. Measurements performed by the
team indicate that the sound field is directional with a
peak acoustic intensity located at an angle that is roughly
45 degrees from the plane of the pressure vessel.
Airborne acoustic measurements made by our group on a
variety of leaks (see Figure 2) show broad acoustic
emission starting at approximately 5 kHz, peaking at
about 40 - 60 kHz, and rolling off at about 160 kHz. This
roll off is due in part to the microphone sensitivity.
The peaks at 170, 140 and 110 kHz appear at lower
pressures and are most likely due to resonances associated
with the aluminum plenum. These peaks are inversely
proportional to frequency and tend to broaden as the
pressure differential increases. In general, the acoustic
emission varies linearly with pressure differential across
the orifice with small changes in the frequency spectrum
as the pressure changes.
Two different methods were identified for detecting the
acoustic emission associated with the leak; on-board
sensors and wayside monitoring. Both methods are
capable of detecting the airborne acoustic emission from
the leak. The on-board method could also include contact
sensors that measure vibrations in the tank walls. Table 1
Power (Relative Units)
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
0
20
40
60
80
100
120
140
160
180
200
Frequency (kHz)
Figure 2. The power spectrum of a variety of leak sizes and shapes from 50 psi to 300 psi. The flat
curve is background noises (no leak).
shows the advantages and disadvantages of each of these
just as easily be mounted on individual tank cars with no
two methods.
effect on the basic performance of the system.
Table 1—Advantages and disadvantages of sensor
locations: on-board the train or near the wayside
Advantages
Sensitive
Disadvantages
Requires
retrofitting many
tank
cars
(~17,000)
Regular
maintenance on
tank cars could be
time consuming
Excluding the air brakes, there is no significant acoustic
energy from the train in the ultrasound regime (>20kHz)
making this frequency regime ideal for detecting tank car
leaks. Several vendors of ultrasound sensors were
contacted and samples from three different vendors were
acquired and tested.
Only detects a
leak when the
train passes by
the sensors
Decreased
sensitivity
as
compared to onboard
The final sensors selected were a pair of piezoelectric
transducers from Massa Products Corporation. The
sensors chosen had peak sensitivities at 40 and 75 kHz
and were approximately 3 orders of magnitude more
sensitive than the ITC sensors. These sensors were
approximately $100 each but would be significantly
cheaper if bought in large quantities. Additionally the
sensors were significantly smaller than the ITC sensors
measuring less than 1 cm in diameter and weighing 6
grams. In comparison the 40 and 85 kHz sensors from
ITC measured 5 cm and 2.5 cm in diameter, respectively
and weighed 196 and 26 grams, respectively. It was
determined that the Massa sensors were the best fit for
this application.
We have focused on wayside monitoring since wayside
systems already exist for measuring wheel and bearing
failures. In this way, the leak detection system could be
integrated with existing systems or at least be modular
enough to compliment these systems. The system could
In order to extend the frequency response of the Massa
sensors, 5 mH inductors were added in parallel to each
sensor creating an LC resonant circuit which increased the
sensitivity beyond the mechanical response of the system.
As a result the transducers had additional sensitivity at 55
kHz in addition to their mechanical response. This yielded
On Board
Sensors
Wayside
Sensors
Constant
leak
surveillance
Smaller number
of installations
than
contact
method
Sensors can be
placed
in
strategic
locations
near complete ultrasound coverage from 35 kHz to 80
kHz.
3. SYSTEM DESIGN
A 10 m tall tower houses the system position
approximately 10 m from the center of a rail line and
siding located in Bowling Green, KY. The current
position is due to concerns with the railroad right-of-way
but future installations will be within a couple of meters
of the rails.
Our data indicates that a sound pressure level of below 40
dB can be detected at distances up to 10 m for sound
sources with a frequency of approximately 40 kHz. This
frequency band is relatively quiet which allows detection
at these low sound levels. Figure 3 compares the power
spectrum of a narrow band transducer (center frequency
of 40 kHz) of a train (blue), a 100 psi leak through a 78
m diameter hole (black), and an electronic surrogate of
the sound made by the hole (red).
All of the
measurements were made at 10 m from the transducer.
The electronic sound source is used in testing near the
railroad tracks.
As described earlier, the system has five transducers, two
40 kHz center frequency and three 75 kHz center
frequency transducers. To increase sensitivity, parabolic
dishes are used as shown in Figure 4.
A small
microphone is used to normalize the sound levels.
In order to monitor the trains for special nuclear materials
and other radiological materials, a 5 cm x 5 cm NaI(Tl)
gamma ray spectrometer has been added to the system.
Spectra are collected every 60 seconds and saved to disk
as the train passes by.
There are three video cameras. As shown in Figure 4,
two of these cameras view the approaching trains from
Figure 4. A train passing the rail car leak detection
system located on the tower in the center. Four
acoustic sensors (parabolic dishes) can be seen. A
video camera is mounted to inspect the passing train.
The large metal box at the bottom of the tower houses
the data acquisition system and the WiFi access point.
The radiological sensor (not shown) is typically
located just above the lowest acoustic sensor. The
tower is located on university property and does not
infringe on the rail-road right-of-way.
different directions. The third camera is aimed to view
the passing train and its images are stored on disk.
The sampling frequency of the ADC is 200 kSamples/s
and thus the Nyquist frequency is 100 kHz. Power
spectra are taken every 20 ms and averaged over 5 video
frames of this third camera. The averaging helps smooth
transient noise and the resulting spectra are stored within
the AVI file with the video data. Thus every frame has an
audio spectrum associated with it.
Figure 3. Power spectrum (amplitude is volts2) of a
train (blue), a 100 psi leak from a 78 mm hole, and an
electronic sound source simulating the leak. See text
for details.
The system is controlled by a website and server located
at the tower and the Internet connection utilizes WiFi
(802.11g) radios transmitted to the Western Kentucky
University (WKU) campus.
During the inspection
process, the train moves past the tower at normal speeds.
An infrared sensor triggers the data collection.
4. DATA ANALYSIS
Figure 5 shows the user interface for the data analysis.
The upper left corner of the interface is a video player
which allows the user to play the video in real time or to
step through the video of the train a frame at a time. In
the upper right corner is an audio spectrogram. As each
frame is shown in the video, the spectrogram will advance
in time. The corresponding power spectrum for the
transducers is shown in the lower left (40 kHz center
frequency transducers) and the lower right (75 kHz center
frequency transducers). The power spectrum shown in
Figure 5 at the lower left is the electronic sound source in
the presence of the train. The source and the train are at
the same distance from the transducers. The electronic
sound source only produces a 35-40 kHz spectrum which
is why there is no signal in the 75 kHz transducers in the
lower right. Currently, detection is made through visual
inspection of the power spectrum.
5. CONCLUSION
Based on our research, we have shown that using
ultrasonic sensors, one can detect the leaks in pressurized
rail cars. The minimum sound level detectable is less
than 40 dB at a distance of approximately 10 m. This has
allowed us to build a monitoring system for these tank
cars as they pass at normal speeds. Orthogonal sensors
such as video cameras, allow us to visually inspect the
passing cars as well. Since the video frames are time
correlated to the audio spectra, we can inspect, one frame
at a time, both visually and audibly, the passing cars. A
third orthogonal sensor (a radiological detector) is also
correlated to the video frames in order to increase our
threat or hazardous material detection capability.
Further work to be done is to 1) automate the audio
detection algorithms, 2) correlate the radiation spectrum
with the video image, and 3) create receiver-operator
characteristic (ROC) curves to evaluate the system’s
effectiveness.
6. ACKNOWLEDGMENTS
This work was supported by the Department of Homeland
Security under contract EKU 06-203 and supported in
part by the Applied Research and Technology Program of
Western Kentucky University. We also wish to thank the
National Institute of Hometown Security for their support
during this project.
Figure5. Data analysis interface for the tank car leak detection system. See text for a complete description.
REFERENCES
[1] Edward R. Hamberger, President/CEO Association of
American Railroads, Statement before the US House of
Representatives Committee on Transportation and
Infrastructure, Subcommittee on Railroads, June 13, 2006.
[2] Hazardous Materials Information System, US
Department of Transportion. Data as of 9/7/2006
[3] United States General Accounting Office, Report to
Congressional Requesters, “Rail safety and security:
Some actions already taken to enhance rail security, but
risk-based plan needed,” April 2003.
[4] R. G. Edmonson, “Not in my backyard: Railroads,
shippers resist efforts by cities to require rerouting of
hazmat shipments,” Journal of Commerce, July 24, 2006.
[5] J. Riley, “Terrorism and Rail Security,” Testimony
presented to the Senate Commerce, Science, and
Transportation Committee, RAND Corporation, March
23, 2004.
[6] Accessed at
http://www.vpc.org/graphics/whyregulate50s.pdf in
November 2006.