Curves and cant

A train on a curve needs an inward force to follow it. Without cant, all of that force would come from the flanges pressing on the outer rail, causing wear, noise and discomfort. Raising the outer rail above the inner one, called cant or superelevation, tilts the vehicle so that a component of its weight provides the inward force.

Cant can be exactly right for only one speed, the equilibrium speed. Faster trains feel an outward unbalanced acceleration; the amount of extra cant they would need is the cant deficiency. Slower trains, often heavy freight, lean inwards and load the inner rail; the excess of actual cant over their equilibrium cant is the cant excess. On mixed-traffic lines, cant is a compromise between fast passenger trains and slow freight, and both deficiency and excess are limited. Indian Railways' manual sets the maximum cant and permitted deficiency by route group.

Cant is run in and out gradually along transition curves, in which the curvature also grows from zero to its full value, so that the train's lateral acceleration and the rate of change of cant stay within comfort and safety limits. Lengths of transitions are set by rate of change of cant, rate of change of deficiency and the cant gradient.

Curves also need checks on gauge widening, check rails on sharp curves, and the clearance between adjacent tracks and structures, since a canted vehicle leans.