AskDefine | Define derail

Dictionary Definition



1 cause to run off the tracks; "they had planned to derail the trains that carried atomic waste"
2 run off or leave the rails; "the train derailed because a cow was standing on the tracks" [syn: jump]

User Contributed Dictionary



From de- + rail.


  • /dəˈreɪl/ or /ˌdiːˈreɪl/
  • Rhymes with: -eɪl


  1. To cause to come off the tracks.
    The train was destroyed when it was derailed by the penny.
  2. To come off the tracks.
  3. To deviate from the previous course or direction.
    The conversation derailed once James brought up politics.
  4. To cause to deviate from a set course or direction.
    The protesting students derailed the professor's lecture.

Derived terms


to come off the tracks
to deviate from the previous course or direction
to cause to deviate from a set course or direction

Extensive Definition

A Derail or Derailer is a device used to prevent fouling of a track by unauthorized movements of trains or unattended rolling stock. It works (as the name suggests) by derailing the equipment as it rolls over or through the derail.
Operating a train over a derail will usually result in a derailment. However, derailments can also occur accidentally, without the use of a derailer. Under normal railroad operating practice, a derailment is an undesirable operation because it will result in damage to the derailed vehicle and expense to restore it back onto the track; it would be the equivalent of a vehicle accident in the use of trucks or a crash in the case of airplanes. The use of a derail is a special case where it is extremely likely that if the rolling stock in question is not derailed at that point, worse damage to equipment and potential injury or death may occur than if it is.
Derails may be applied:
  • where sidings meet main lines or other through tracks
  • at junctions or other crossings to protect the interlocking against unauthorized movement
  • at an area where crews are working on a rail line (via a portable derail device)
There are three basic forms of derail. The most common form is a wedge-shaped piece of steel which fits over the top of the rail. If a car or locomotive attempts to roll over it, the wheel flange is lifted over the rail to the outside, derailing it. When not in use, the derail folds away, leaving the rail unobstructed. It can be manually or remotely operated; in the former case it will have a lock applied to prevent it from being moved by unauthorized personnel. This type is common on North American railroads.
The second type of derail is known as the 'Split Rail' type (termed trap points or catch points in the UK). These are basically a complete or partial railroad switch which directs the errant rolling stock away from the main line. This form is common throughout the UK.
The third type of derail is the Portable Derail, and is used by railroad mechanical forces, as well as some industries. Often used in conjunction with Blue Flag rules and are temporary in nature.
In North American practice, the normal position of a derail is in the derailing position (i.e. applied or left on).
Derails have failed on occasion, such as on April 1, 1987 at Burnham, Illinois when an unsecured car in a siding defeated a derail and fouled the mainline. Due to rusty rails, the car failed to shunt the track circuit and put block signals to 'stop'.

See also


derail in Czech: Výkolejka
derail in German: Gleissperre
derail in Polish: Wykolejnica
derail in Finnish: Raiteensulku
derail in Hebrew: שומטן
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