Hinged derail with automatic blue flag

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, or a train) by unauthorized movements of trains or unattended rolling stock.[1] 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.


A derailed train car at a closed siding

Derails may be applied:


A derail device installed on a siding at Glen Haven, Wisconsin, oriented to protect track located off the bottom of the picture

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.[7] 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.[8] 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.[9] This type is common on North American railroads.[10]

Derail on Nemuro Main Line, JR Hokkaido. It is set so that a train proceeding away from the camera will remain on the track.

Split rail

The second type of derail is the "split rail" type.[7] 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.

Split rail type derail on the A&M in Rogers, Arkansas


The third type of derail is the portable derail, and is used by railroad mechanical crews, as well as some industries. This is often used in conjunction with blue flag rules (meaning equipment on the track must not be moved, as workers are on or near the equipment) and is temporary in nature.[6] 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 (49 CFR 218.109.).


Powered derailer, Temple Mills, Eurostar Maintenance Depot, UK

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. Examples include:

See also


  1. ^ Kellenberger 1919, p. 208.
  2. ^ a b Kellenberger 1919, p. 203.
  3. ^ 49 CFR 213.357
  4. ^ a b Adams & Hitt 1912, pp. 195–197.
  5. ^ Meyer, J. Joe (March 9, 1978). "Portable derail". US Patent & Trademark Office. Archived from the original on May 22, 2018. Retrieved September 25, 2006.
  6. ^ a b 49 CFR 218.29
  7. ^ a b Kellenberger 1919, p. 206.
  8. ^ Hayes Track Appliance Co. (1921). "Derails". In Howson, E.T.; Lewis, E.R.; Kellenberger, K.E. (eds.). Maintenance of Way Cyclopedia. New York: Simmons-Boardman Publishing Co. p. 704 – via Google Books.
  9. ^ Hayes, Stanley W. (August 7, 1917). "Patent 1,235,971, Derail". Official Gazette of the United States Patent Office. Vol. CCXLI. Washington, D.C.: Government Printing Office. p. 145 – via Google Books.
  10. ^ "Railway Safety & Track Repair Products | Aldon®". Aldon Company, Inc. Retrieved 2023-03-07.
  11. ^ https://www.jonroma.net/media/rail/accident/usa/icc/3813-a.pdf
  12. ^ "Faulty coupling, rotted ties led to deadly train derailment in Woss, B.C.: report". CBC News. CBC News.