Retarder (railroad)

In rail transport, a retarder is a device installed in a classification yard used to reduce the speed of freight cars as they are sorted into trains.
Construction.
Each retarder consists of a series of stationary brakes surrounding a short section of each rail on the track that grip and slow the cars' wheels through friction as they roll through them.
Computer control.
Modern retarders are computer controlled to apply a precise amount of pressure on the wheels so that cars rolling down a yard's hump are slowed to a safe speed for coupling with cars already standing on the yard's tracks.
Inert retarder.
An inert retarder holds a cut of classified railcars to keep them from rolling out of a yard.