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.

Rerailing
Most railway accidents involve derailment. See list of rail accidents.

November 11, 1833 – Hightstown, New Jersey, United States: Carriages of a Camden & Amboy train derails at 25 miles per hour in the New Jersey meadows between Spotswood and Hightstown when an axle breaks on a car due to an overheated journal. One car overturns, killing two and injuring fifteen. Among the survivors is Cornelius Vanderbilt who will later head the New York Central Railroad. He suffers two cracked ribs and a punctured lung, and spends a month recovering from the injuries. Uninjured in the coach ahead is former U.S. President John Quincy Adams, who continues on to Washington, D.C. the next day.
January 6, 1853 – Andover, Massachusetts, United States: The Boston & Maine noon express, traveling from Boston to Lawrence, Massachusetts, derails at forty miles an hour when an axle breaks at Andover, and the only coach goes down an embankment and breaks in two. Only one is killed, the twelve-year-old son of President-elect Franklin Pierce, but it is initially reported that General Pierce is also a fatality. He was on board but is only badly bruised. The baggage car and the locomotive remain on the track.
April 16, 1853 – Cheat River, West Virginia, United States: Two Baltimore & Ohio passenger cars tumble down a hundred foot ravine above the Cheat River in West Virginia, west of Cumberland, Maryland, after they are derailed by a loose rail. Example accidents

December 12, 1917 – Saint Michel de Maurienne, France: A military train derails at the entrance of the Fréjus Rail Tunnel after running away down a steep gradient; brake power was insufficient for the weight of the train. Around 800 deaths estimated, 540 officially confirmed. The world's worst ever derailment, and worst rail disaster up to the end of the 20th century.
July 2, 1922 – Winslow, Camden County, New Jersey, United States: The Owl, a Reading Railroad train derailment, at Winslow Junction on the West Jersey and Seashore Line tracks near the Winslow Tower, shortly before mid-night, Train 33 derails when the seashore bound locomotive going more than 90 miles per hour sped through an open switch. 4 passengers, the engineer, fireman and conductor were killed.
February 18, 1947 – Blair County, Pennsylvania, United States: The Red Arrow, a Pennsylvania Railroad express passenger train, jumped off the track on the Bennington Curve near Altoona, Pennsylvania and tumbled down a large hill. 24 killed, 131 injured.