For example, if a train is to operate around a 2-degree curve at 55 miles per hour, the equilibrium
superelevation would be calculated as follows:
= .00066 x D x V2
= .00066 x 2 x (55)2
= .00066 x 2 x (55 x 55)
= .00066 x 2 x 3025
= 3.99300 or 4 inches
If a train exceeds the speed for which the equilibrium superelevation was determined, a greater force
bears on the high or outside rail than on the low or inside rail. If the speed is lower than that for which the
equilibrium superelevation was calculated, a greater force is exerted on the low rail.
Railway engineers generally agree that a train can travel over a curve with 3 inches less superelevation
than the equilibrium superelevation calculated for the train's speed. In the example just given, the equilibrium
superelevation is 4 inches; but if the elevation were 3 inches less, or 1 inch, the train could still pass around it at
55 mph (88 kmph) without endangering safety or comfort. A superelevation where the high rail is 3 inches lower
than the equilibrium superelevation is said to have 3-inch unbalanced elevation. Such-curves are commonly used
where both slow and fast trains use the same track.
Determining the proper superelevation is a simple matter when all trains operate at approximately the
same speed, but this is seldom true. It is most closely approximated on multiple-track lines having separate
tracks in each direction for freight and passenger trains. Double-track railroads usually assign the tracks by
direction rather than by speed or service. Single-track lines create a greater problem than multiple tracks,
especially where curves occur on grades. Because both passenger and freight trains operate over the same track
in both directions, the expected train speed for a curve will vary between that of passenger trains descending the
grade and that of heavy freights climbing the grade. Use of the 3-inch unbalanced elevation is then necessary.
Quite often this is not enough