CE 515 Railroad Engineering
Railway Track Design
Source: AREMA Ch. 6
Stationing&
Horizontal Alignments
“Transportation exists to conquer space and time -”
Railway Track Design
Key factors for rail alignment
• Operator has no control over horizontal
movement
• Less relative power available for locomotion
relative to the mass to be moved
• Long and thin physical dimension
Railway Track Design
Considerations for designing rail alignments
• Type of train traffic
• Volume of traffic
• Speed
Stationing
Stationing
• A single station: 100 feet long from beginning
point to end point
Milepost
• Less precise, but more easily identified
Horizontal Alignments
Two components: Tangents and Curves
Tangents: established first
• A straight line between two points
• Denoted with bearings
– Need a starting point and length to locate in space
Curves: established based on tangents
• Point of Intersection (PI) - fixed
• Define the tangents
• Points of curve defined subsequently
Horizontal Curves
Important components of simple circular curve
• Angle of Deflection (I)
• Other aspects
– see figure
Source: AREMA
Degree of Curve
Source: AREMA
Curves: Chord and Arc Definitions
• Differences between two definition are slight at
small degrees of curves
• But increase as curves get sharper
• North America freight railways use chord-defined
– Highway design, some light rail system and other
alignments design use arc-defined
Curves
• Note: the radius of a four-degree curve is not
exactly half of that of a two-degree curve
Source: AREMA
Centrifugal Force
• Can be counteracted
by superelevation
– Equilibrium elevation,
allow more uniform
speed
Source: AREMA
Horizontal Alignments
Under-balance
• Difference between the equilibrium elevation
and the actual superelevation
• Limited to three inches by FRA
Source: AREMA pg.224
Spiral Curves
• Transition from tangent level track to curved
superelevated track
• Two functions
– Gradual introduction of superelevation
– Guiding the train from tangent track to curved
track
Types of Spirals
• Commonly used: the Searles, the Talbot, and
the AREMA 10-chord spiral
• Two Design Criteria
– Rotational acceleration of the train about its
longitudinal axis
– Limiting value of twist along car body
Recommended formula for the minimum length of
spiral on AREMA pg. 225
Questions?