A derail or derailer is a device used to prevent fouling (blocking or compromising) of a rail track (or collision with anything present on the track, such as a person, a train or a fallen branch) by unauthorized movements of trains or unattended rolling stock. The device works by derailing the equipment as it rolls over or through it.
Although accidental derailment is damaging to equipment and track, and requires considerable time and expense to remedy, derails are used in situations where there is a risk of greater damage to equipment, injury or death if equipment is allowed to proceed past the derail point.
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
- temporarily at an area where crews are working on a rail line
- 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 the "split rail" type. 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, where it is called trap points or catch points.
The third type of derail is the portable derail, and is used by railroad mechanical forces, as well as some industries. This is often used in conjunction with Blue Flag rules and is temporary in nature. They are placed onto one side of the rail with the derail pointed to the outside of the track. Then there is a part of the derail that is able to be tightened down to the rail and then secured with a locking mechanism. If the derail is left unlocked for any reason or does not have a locking mechanism deployed then the owner of the derail can face substantial fines if found by an FRA inspector.
The fourth type of derailer is the powered or motorized derailer, electronically powered through an actuator. This type of derailer can be controlled remotely from an external control panel or manually. It is commonly installed as a part of Depot Personnel Protection Systems, to ensure personnel safety in maintenance workshops and depots.
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 then failed to shunt the track circuit that should have put block signals to "stop".
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