In rail transport, a retarder is a device put in a classification yard used to slow down the speed of freight cars as they are sorted into trains. Each retarder has a series of stationary brakes around a short section of each rail on the track that hold and slow the cars' wheels through friction as they roll through them. Modern retarders are computer worked to have the perfect 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.
Retarder Facts for Kids. Kiddle Encyclopedia.