derailment
The action of a locomotive or train leaving the rails along which it runs.
derailment: train leaves the track unplanned
A derailment occurs when one or more wheels of a locomotive or rail vehicle lose contact with the rail head and climb or slip sideways off the track structure. The vehicle may remain upright on the ballast and sleepers, tip onto its side, or overturn entirely depending on speed, load, and track geometry at the moment of failure. Derailments range from low-speed incidents where a train can be re-railed with minimal damage to high-speed wrecks that destroy rolling stock and infrastructure.
The most common causes are track defects, inadequate maintenance, or operational error. Corroded or broken rails, spread gauge (rails pushed wider than standard spacing), or worn wheel flanges that no longer grip the rail head can trigger derailment at normal operating speeds. Sharp curves taken too fast, especially with heavy loads, can cause outer wheels to climb the high rail. Debris, ice, or snow packed between rails can act as a wedge. Broken wheels or axles fail suddenly and catastrophically.
Detection and consequences
Modern railways use wayside detectors to identify hot axle bearings, dragging equipment, and wheel defects before they cause derailment. Derail devices (portable or permanent) are placed on sidings or at grade crossings to force wheels off the rail in a controlled manner if an unwanted movement occurs, preventing a runaway into occupied track. After a derailment, investigation focuses on track geometry, wheel condition, speed data from event recorders, and load distribution to determine the root cause and prevent recurrence.
The term reflects the literal meaning: the train has left (de) the rail (railer, from Old French raille). In railway engineering, rail originally referred to the wooden or iron guidance structure itself, not the entire track system. A derailment is classed by severity as minor (low speed, easy recovery), major (speed or damage), or catastrophic (loss of life or major infrastructure damage). Speed, tonnage, and track class all influence whether a given defect results in a derailment or merely rough riding.