Abstract: A modular control system for a railroad classification yard is described. The control system can automatically perform those functions necessary to control the various elements of a railroad classification yard to enable the train of cars to be switched from a hump track to one of a plurality of bowl tracks in accordance with the destination for the car. The control system comprises a number of subsystems including a hump control system (HUMPCON), an operator communications subsystem (OPCOM), a switching control subsystem (MASC), a retarder control subsystem (MARC), a multidrop communications system (MDCOM), a crest monitor subsystem (CMON) and a distance to couple subsystem (MADTC). Some of the subsystems are implemented as singular modules (HCON, OPCOM, CMON). Other subsystems include multiple modules (MARC, MASC, MDCOM). Each module in each subsystem is comprised of a single microprocessor and related peripheral circuits.