Source : Webster's Revised Unabridged Dictionary (1913)

Derail \De*rail"\, v. t. [imp. & p. p. {Derailed}; p. pr. & vb.
   n. {Derailing}.]
   To cause to run off from the rails of a railroad, as a
   locomotive. --Lardner.

Source : WordNet®

derail
     v 1: cause to run off the tracks; "they had planned to derail the
          trains that carried atomic waste"
     2: run off or leave the rails; "the train derailed because a
        cow was standing on the tracks" [syn: {jump}]