Definition of "derail"

derail

/ˌdəˈɹeɪ.əl/

noun
plural derails
1. Synonym of derailer: A device placed on railway tracks in order to cause a train to derail.
2. An instance of diverting a conversation or debate from its original topic.
verb
third-person singular simple present derails, present participle derailing, simple past and past participle derailed
1. (transitive) To cause to come off the tracks.
  Quotations
  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.
  Quotations
  Fortunately, the CrossCountry train did not derail when it struck the mud. It could easily have been much worse.
  2020 September 9, 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.
4. (transitive, figurative) To cause to deviate from a set course or direction.