What is railroad derailleur?

It is not clear what is meant by "railroad derailleur" as the term "derailleur" typically refers to a mechanical device used on bicycles to shift gears.

However, in the context of railroads, a derail or derailment device is a mechanism used to prevent trains from accidentally rolling onto a side track or off the tracks altogether.

A derail typically consists of a metal plate or blade that can be lowered onto the tracks, causing the train's wheels to be displaced from the rails and forcing it to come to a stop. Derails are usually positioned at strategic locations such as near switches or on steep inclines where a runaway train could be particularly dangerous.

Derails can also be activated remotely by railroad personnel or designed to be "self-setting" if a train passes over them incorrectly. They are an important safety feature of railroads, helping to prevent accidents and protect people and infrastructure from harm.