Derailment

From Wikinfo

A derailment, generally of a train traveling on a railway, is the failure of a vehicle (or other mechanism) to remain on the track or fixed path laid out for its movement. Derailments of a railway train can result in substantial loss of life both from wreck of the train itself, but also from hazardous materials release. Derailment may occur due to faulty equipment or defects in the railroad track or due to collision with something accidently or deliberately placed on the track. Grade crossing collisions sometimes result in derailments.

Derailments can be much more mundane, for example a sliding window or patio door, may become derailed as may any similar mechanism.