derailment

noun

1. (countable, uncountable)The action of a locomotive or train leaving the rails along which it runs.

   “The length ganger saw the train passing with the van derailed and promptly telephoned the Sandbach signalman, who restored his signals to danger, but not in time to stop the train before the final derailment occurred.”

   “In retrospect, it was small wonder that Railtrack found its finances under pressure, as with ever increasing demand there was an inevitable effect on infrastructure renewals. Matters came to a head with the Hatfield accident on October 17 2000, when there was a high-speed derailment as a result of deferred track maintenance.”

2. (countable, uncountable)An instance of diverting a conversation or debate from its original topic.
3. (countable, uncountable)An instance of thwarting or frustrating something.

   “But when the second stage set in, in which man became preponderantly self-conscious, he inevitably set about deflecting sex-activities to his own private pleasure and advantage; he employed his budding intellect in scheming the derailment of passion and desire from tribal needs and, Nature's uses to the poor details of his own gratification.”

4. (countable, uncountable)A pattern of discourse (in speech or writing) that is a sequence of unrelated or only remotely related ideas.