Derail

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Derail device installed at an industry, complete with Blue Flag protection -- Nichols, Florida.

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)^[1]

A derail device installed on a siding at Glen Haven, Wisconsin.

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.

Derail on Nemuro Main Line, JR Hokkaido. It has currently been set that a train coming from the position of the camera will remain on the track.

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.

A derail in Mannheim, Germany

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'.