A derailer is a mechanical protection device on railroad tracks. It prevents rail vehicles from driving on the track.

They are used in front of switches in sidings, junctions and for the protection of certain track systems, whose track runs directly into another (cf https://bahnblogstelle.net/lexikon/gleissperre/). On the one hand, there are derailers that are switched on site by the shunting personnel, and on the other hand, there are remote-controlled derailers that can be operated remotely from an interlocking.