Retarder

A retarder is a track-mounted braking device used to control the speed of freight cars as they are sorted in a hump yard. It acts as a linear brake that presses against the wheels as a car rolls through it, slowing the car to a controlled speed without any action by an onboard crew. The most common type is the clasp (side) retarder, which grips the sides of the wheels; other designs act on the wheel tread.

How It Works

In a hump yard, cars are shoved over a small hill and then released to roll by gravity down onto their assigned classification yard tracks. Left uncontrolled, cars would roll at widely varying speeds depending on their weight, the wind, and rolling resistance. Retarders solve this by gripping the wheels as the car passes through the retarder group and bleeding off excess speed. The amount of braking is adjusted for each car based on the car's weight, the track it is being routed toward, and how far down that track it must roll to couple safely with the cars already standing there.

Why It Matters

Proper speed control protects both the equipment and the lading. A car released too fast can strike standing cars hard enough to cause damage, while a car that is slowed too much may stall short of its coupling. By metering each car's speed, retarders allow a hump yard to sort large volumes of cars quickly while keeping coupling impacts within safe limits. In modern yards, retarder operation is managed by the same automated control system that lines the routing switches.