A derail or derailer is a device used to prevent fouling of a track

Rail tracks

The track on a railway or railroad, also known as the permanent way, is the structure consisting of the rails, fasteners, sleepers and ballast , plus the underlying subgrade...

 by unauthorized movements of trains or unattended rolling stock

Rolling stock

Rolling stock comprises all the vehicles that move on a railway. It usually includes both powered and unpowered vehicles, for example locomotives, railroad cars, coaches and wagons...

. It works (as the name suggests) by derailing the equipment as it rolls over or through the derail.

Although accidental derailment is damaging to equipment and track, and requires considerable time and expense to remedy, derails are used in situations where allowing equipment to proceed risks greater damage to equipment and potential injury or death may occur than if it is derailed at the spot.

Derails may be applied:

• where sidings
  Rail siding
  A siding, in rail terminology, is a low-speed track section distinct from a running line or through route such as a main line or branch line or spur. It may connect to through track or to other sidings at either end...
  
   meet main lines or other through tracks
• at junctions or other crossings to protect the interlocking
  Interlocking
  In railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. The signalling appliances and tracks are sometimes collectively referred to as an interlocking plant...
  
   against unauthorized movement
• at an area where crews are working on a rail line (via a portable derail device)
• approaching a drawbridge, dead end, or similar hazard.

There are four 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

Railroad switch

A railroad switch, turnout or [set of] points is a mechanical installation enabling railway trains to be guided from one track to another at a railway junction....

 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.
The fourth type of derailer is the Powered or Motorized Derailer, electronically powered through an actuator this type is of derailer could be controlled remotely from an external control panel or manually. It is commonly installed as a part of Depot Personnel Protection System

Depot Personnel Protection System

A Depot Personnel Protection System, commonly referred to as Depot Protection System or DPS, is a system designed to protect staff and equipment by ensuring the safe and controlled movement of rail vehicles into and out of train maintenance depot, a process sometimes known as shunting...

s installed to ensure personnel safety in maintenance workshops and depots.

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 the Newark Bay rail accident of 1958, or on April 1, 1987 at Burnham

Burnham, Illinois

Burnham is a village in Cook County, Illinois, United States. The population was 4,170 at the 2000 census. The ZIP code is 60633.Burnham has a Chicago ZIP code, and is often mistakenly considered part of Chicago...

, Illinois

Illinois

Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

 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

Track circuit

A track circuit is a simple electrical device used to detect the absence of a train on rail tracks, used to inform signallers and control relevant signals.- Principles and operation :...

and put block signals to 'stop'.