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MODERN classification yards permit an incoming freight train to be redistributed to tracks assigned to various destinations, with all measurements, computations, and speed control performed automatically. In such yards cars are uncoupled at the crest of a hill and then accelerate freely down a slope which is usually about 3%. This increase in speed separates the cars so that track switches may be safely thrown between cars. Some 250 feet down the hill is the master retarder, a pneumatic or electric brake, which squeezes the wheels of each car between brake beams and reduces the car speed in a controlled manner to a value which can be handled by subsequent retarders. Below the master retarder the tracks fan out to a number of group retarders; here the exit speed must be computed which will allow each car just sufficient energy to traverse the subsequent curve and travel the free roll distance down straight classification track to a gentle coupling with cars already there. A radar-controlled servo loop is closed around each retarder so that the desired exit speed is accurately attained.