Derailment

n. act of running off the rails

For the definition of derailment in psychiatry, see thought disorder. A derailment is an accident on a railway whereby a train leaves the rails, which can result in damage, injury, and death.There are several main causes of derailment: broken or misaligned rails, excessive speed, and faults in the train and its wheels. Derailment can also occur as a secondary effect in the aftermath of a collision between two or more trains. Trap points protect main lines from runaway vehicles by deliberately derailing them to bring them to a stop. Flangeless wheels make it easier for a locomotive to negotiate curves, but make them more prone to derailment.
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Derailment /de·rail·ment/ disordered thought or speech characteristic of schizophrenia and marked by constant jumping from one topic to another [more]Derailment - Community and Resources

Noun
1. an accident in which a train runs off its track
(hypernym) mishap, misadventure, mischance
(derivation) derail

(n.)
The act of going off, or the state of being off, the rails of a railroad.