A derail or derailer is a device used to prevent fouling of a rail track by unauthorized movements of trains or unattended rolling stock. The device works by derailing the equipment as it rolls over or through it.

In rail transport, a derailment is a type of train wreck that occurs when a 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.