Russian Experiments
In the 1960s in the Soviet Union curve resistance was found by experiment to be highly dependent on the banking of the curve, also known as superelevation. If a train car rounds a curve at "balancing speed" such that the component of centrifugal force in the lateral direction (towards the outside of the curve and parallel with the plane of the track) is equal to the component of gravitational force in the opposite direction there is very little curve resistance. At such "balancing speed" there is zero "cant deficiency" and results in a "frictionless banked turn"). But deviate from this speed (either higher or lower) and the curve resistance increases due to the unbalance in forces which tends to pull the vehicle sideways (and would be felt by a passenger in a passenger train ).
There doesn't seem to be any reasonably accurate formulas for curve resistance and it seems that the best we have is old Russian curves from experimental testing. Unfortunately, these experiments were all done on a test track with the same curvature (radius = 955 meters) and it's not clear how to account for curvature. The Russian experiments plot curve resistance against velocity for various types of railroad cars and various axle loads. The plots all show smooth convex curves with the minimums at balancing speed where the curve slope is zero. The plots tend to show curve resistance increasing more rapidly with decreases in velocity below balancing speed, than for increases in velocity (by the same amounts) above balancing speeds. No explanation for this "asymmetrical velocity effect" is to be found in the references cited nor is any explanation found explaining the smooth convex curve plots mentioned above (except for explaining how they were experimentally determined).
Read more about this topic: Curve Resistance
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