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.
- "Safety at the Switches". Popular Science Monthly. New York: Popular Science Publishing Co. 107 (4): 38. October 1925.
- Boyd, Jim (2001). The American Freight Train. MBI Publishing. p. 63. ISBN 978-0-7603-0833-2.
- U.S. Federal Railroad Administration, Washington, D.C. (1983). "Definitions: Inert retarder." Railroad Noise Emission Compliance Regulations. Code of Federal Regulations, 49 CFR 210.5.