Real Dictionary

derail
verb: If a train is derailed or if it derails, it comes off the track on which it is running.
verb: To derail something such as a plan or a series of negotiations means to prevent it from continuing as planned.

derail in American English
to cause (a train, etc.) to go off the rails
to go off the rails
intransitive verb: to run off the rails of a track
noun: a track device for derailing rolling stock in an emergency
transitive verb: to cause (a train, streetcar, etc.) to run off the rails of a track

derail in British English
noun: a device designed to make rolling stock or locomotives leave the rails to avoid a collision or accident
verb: to go or cause to go off the rails, as a train, tram, etc