Derailment

Derailment takes place when a rail vehicle such as a train comes off its rails. Although most derailments are minor, all result in temporary damage of the proper operation of the railway system and they are a potentially serious danger.

A derailment of a train can be caused by a crash with another object, an operational error, the mechanical failure of tracks, such as broken rails, or the mechanical failure of the wheels. In emergency situations, on purpose derailment with derails or catch points is sometimes used to stop a more serious accident from taking place.
 References 

Category:Railway accidents