A railcar retarder assembly for use in a railroad trackwork such as a railcar classification yard is provided with a fixed base plate, a vertically-oriented pivot shaft secured to the base plate, an elongated and curved cam member mounted on the pivot shaft, and a horizontally-oriented hydraulic subsystem piston element that induces reaction hydraulic damping forces which are transmitted horizontally to the wheel flanges of a passing railcar to thereby reduce the rolling velocity of the railcar.