The ACMUs (short for '' Air Conditioned Multiple Unit'') were a series of DC electric multiple unit railcars built for the New York Central railroad in three batches between 1950 and 1965, with a fourth batch supplied to the Long Island Rail Road as the class MP75 in 1963. The ACMUs were intended to replace/supplement the original fleet of heavyweight MU cars, some of which dated to the start of electric operations on both railroads. These were the first New York Central units to offer air conditioning, with sealed windows replacing the drop sash and clerestory types found on earlier cars. The initial set of 100 cars was retired in 1970, upon completion of the M1 railcar delivery, while the second and third batches, totaling 87 cars, remained in service until 2004, when they were replaced by the new M7 fleet. The Long Island cars were eventually converted to non-powered push-pull coaches before being retired in the late 1990s.