cause to run off the tracks; "they had planned to derail the trains that carried atomic waste"
run off or leave the rails; "the train derailed because a cow was standing on the tracks"
a strategically located safety device that is designed to provide secondary protection against uncontrolled movements of train equipment onto the main track
(verb) to leave the rails.
A track structure designed to derail rolling stock in case of an emergency (say for example, a cut of cars rolling down an industrial spur to a mainline track would likely derail upon hitting the turnout at the mainline. A derail set ahead of this point would derail the cars before reaching the mainline reducing the risk of damage to mainline equipment and trackage).
A track safety device designed to guide a car off the rails at a selected spot as a means of protection against collisions or other accidents.
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 if it attempts to roll past the derail.