Derailer may refer to:
* A railroad
derail
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. ...
, also called a ''derailer'', used to prevent fouling of a railroad track
*
Derailleur gears
Shimano 600 front derailleur (1980)
A derailleur is a variable-ratio bicycle gearing system consisting of a chain, multiple sprockets of different sizes, and a mechanism to move the chain from one sprocket to another.
Modern front and rear ...
, a variable-ratio transmission system commonly used on bicycles
{{Disambig