Back to Page 35: Siskiyou line curvatureSuperelevation runoff
The term describes the transition of a superelevated rail to normal. It is best shown with a picture. Just click the following small version for a magnified picture, and see the different angulation of cars on different sections of a superelevation runoff.
If the superelevation runoff is too short, the result is a high vertical acceleration, especially at speed. Railroad carriages are elastic, and can twist, but this ability has limits. Very short runoffs could therefore create a geometrical situation, that is beyond this ability. Derailment probability is high under such circumstances. Due to these concerns, there are safety rules for the minimum length of the runoff.
US safety rules allow 7 inches of superelevation. After substracting a safety margin, 6 inches are usable under real-world conditions. The following table lists minimum lengths for running off 6 inches. In practical railroad operation, some safety margin has to be added to these figures.
"Short spiral situation" points to an exception in the FRA regulations, which allows slightly shorter runoffs for old track restricted to short spirals. ("Examples of short spiral situations include rock cuts, tunnels, station platforms, etc. ... Railroads are expected to apply the variation parameter and thresholds only at locations where there is a clear history of restrictive physical characteristics." FRA Track Compliance Manual, 5.53.)
| passenger train speed | minimum superelevation runoff | minimum spiral length |
|---|---|---|
| 30 - 60 mph (48 - 97 km/h) | 186 feet (57 m) | 132 feet (40 m) |
| short spiral situation | 149 feet (45 m) | 105 feet (32 m) |
| up to 79 mph (127 km/h) | 213 feet (65 m) | 168 feet (51 m) |
| short spiral situation | 186 feet (57 m) | 147 feet (45 m) |
| up to 90 mph (145 km/h) | 248 feet (76 m) | 207 feet (63 m) |
| short spiral situation | 248 feet (76 m) | 207 feet (63 m) |
| higher speeds | 248 feet (76 m) | 248 feet (76 m) |
Whenever possible, superelevation runoff should be as long as the spiral. Under geometrical restrictions, small amounts of runoff can be extended into adjacent straight track. This is the reason for different runoff and spiral figures in the table above.