derail

Short definition from Wiktionary

Noun

1. 1
   Synonym of derailer: A device placed on railway tracks in order to cause a train to derail.
2. 2
   An instance of diverting a conversation or debate from its original topic.

Verb

1. 1
   To cause to come off the tracks.
2. 2
   To come off the tracks.
3. 3
   To deviate from the previous course or direction.
4. 4
   To cause to deviate from a set course or direction.