derail

Etymology

From French dérailler (“to go off the rails”). Analyzable as de- + rail.

noun

1. Synonym of derailer: A device placed on railway tracks in order to cause a train to derail.
   The derail was placed deliberately so that the train would fall into the river.
2. An instance of diverting a conversation or debate from its original topic.

verb

1. (transitive) To cause to come off the tracks.
   The train was destroyed when it was derailed by the collision.
   Among recommendations arising out of the accident were that greater attention should be devoted to the means of derailing runaways on lines so heavily graded as the Highland main line; were it double throughout, catch points would, of course, be laid in, but the catchpoint problem is a difficult one on a single line. 1940 November, “Notes and News: A Highland Collision”, in Railway Magazine, page 612
2. (intransitive) To come off the tracks.
   Fortunately, the CrossCountry train did not derail when it struck the mud. It could easily have been much worse. September 9 2020, Paul Clifton, “Heavy rainfall causes landslip in Hampshire: At the scene...”, in Rail, page 10
3. (intransitive, figurative) To deviate from the previous course or direction.
   The conversation derailed once James brought up politics.
4. (transitive, figurative) To cause to deviate from a set course or direction.
   The protesting students derailed the professor's lecture.