In rail transport

Rail transport

Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...

, a retarder is a device installed in a classification yard

Classification yard

A classification yard or marshalling yard is a railroad yard found at some freight train stations, used to separate railroad cars on to one of several tracks. First the cars are taken to a track, sometimes called a lead or a drill...

 used to reduce the speed of freight cars as they are sorted into train

Train

A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...

s. Each retarder consists of a series of stationary brakes surrounding a short section of each rail on the track

Rail tracks

The track on a railway or railroad, also known as the permanent way, is the structure consisting of the rails, fasteners, sleepers and ballast , plus the underlying subgrade...

 that grip and slow the cars' wheels through friction

Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

as they roll through them. 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.

An inert retarder holds a cut of classified railcars to keep them from rolling out of a yard.