A derailment is an incident on a railway or tramway in which one or more rail vehicles leave the tracks on which it is, or they are, travelling. A derailment is defined as an event in which the proper guidance of railway vehicles is disrupted.
There are several principal causes of derailment: broken or misaligned rails, excessive speed (especially on curves), faults in the train and its wheels, and faults in sets of points. Derailment can also occur as a secondary effect in the aftermath of a collision.
Trap points and derails are designed to protect main lines from runaway vehicles by deliberately derailing them, preventing them from fouling running lines; improper movement through them may derail a train or vehicles.
Read more about Derailment: Rail Breakages, Misaligned Railroad Tracks, Slow-speed Derailments, In-train Forces, Wheel and Truck Failures, Obstacles, Earthquakes, Sabotage, Rerailing