A Derail or Derailer is a device used to prevent fouling of a track by unauthorized movements of trains or unattended rolling stock. It works (as the name suggests) by derailing the equipment as it rolls over or through the derail
run off or leave the rails; "the train derailed because a cow was standing on the tracks" cause to run off the tracks; "they had planned to derail the trains that carried atomic waste
If a train is derailed or if it derails, it comes off the track on which it is running. Several people were injured today when a train was derailed No-one knows why the train derailed
To derail something such as a plan or a series of negotiations means to prevent it from continuing as planned. The present wave of political killings is the work of people trying to derail peace talks