derailment

In rail transport, a derailment is a type of railway accident that occurs when a moving rail vehicle such as a train comes off its rails. Although many derailments are minor, all result in temporary disruption of the proper operation of the railway system and they are a potentially serious hazard.

A derailment of a train can be caused by a collision with another object, operator error (such as excessive speed through a curve), mechanical failure of tracks (such as broken rails) or wheels, among other causes. In emergency situations, deliberate derailment with derails or catch points is sometimes used to prevent a more serious accident.