I have this picture of a retarder at the bottom of the hump yard. It was taken at the BNSF yard in Barstow, CA. Can anyone explain how thse work?
Your photograph is not clear enough to tell what type of retarder that is. There are basically two types. One applies pressure to the backs of the wheels, like a disc brake. The newer type, a Dowty retarder, is a series of raised "buttons" that act on the wheel tread, compressing and effectrively "flattening out" the hump.
