What is a Dowty Retarder?
A Hydraulic device that regulates the speed of rail cars as they roll down an inclined track, such as marshalling yards.
The devices are built for a specified speed set at the factory.
If the rail car is below the set speed, the device offers no resistance. If the rail car speed is at or greater than the speed setting, an internal valve is activated to provide resistance to the rail car wheel, thus slowing the rail car.
This keeps rail cars rolling within a relatively narrow range of speeds
Film which includes a section on the Dowty Retarder