Definition of "derailed"

Sparks from the derailed bogie of the train were first noticed by the signalman at Slough West box, who immediately sent to Slough Middle box the "Stop and Examine" signal, followed at once by "Obstruction Danger" when he realised that the coach was derailed.

1960 March, “Talking of Trains: The Slough derailment”, in Trains Illustrated, page 132