When a train or a subway derails, its wheels accidentally leave the tracks. Train cars that derail can cause the cars behind them to crash.

Mistakes by engineers or flaws in the track can derail a train or trolley, sending it off the rails and sometimes injuring the passengers. You can also use the verb derail figuratively, to mean "hinder someone" or "divert a process from going in its intended direction." For example, terrorists might derail ongoing peace talks by blowing up a bus. The word comes from the French dérailler, "go off the rails."

Definitions of derail

v run off or leave the rails

“the train derailed because a cow was standing on the tracks”
Type of:
go, locomote, move, travel
change location; move, travel, or proceed, also metaphorically

v cause to run off the tracks

“they had planned to derail the trains that carried atomic waste”
Type of:
counteract, countermine, sabotage, subvert, undermine, weaken
destroy property or hinder normal operations

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