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 the New York Central.

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 ACMU fleet was intended to replace some of the oldest MU cars on the New York Central system, many of which were already 40 years old and dated from the start of electrified service in 1906.

With the railroad's finances worse than ever, the state acted to further prop up the ailing commuter operations and its fleet of several hundred remaining pre-war MU cars through the purchase of 174 new M1A "Metropolitan" railcars which were delivered between 1971 and 1973.

In 1983, Metro-North took over operations from Conrail and embarked on a capital improvement program that included the installation of cab signals on its main lines.

In addition to air conditioning and large sealed windows, the cars had flush steel panel sides with smooth rounded vents.

The propulsion systems were significant upgrades featuring camshaft driven motor controllers that offered both higher and smoother acceleration.

The air compressors and motor gear train were far quieter; that along with the sealed windows and fluorescent lighting provided an isolated and relaxing environment for the passengers.

After the electrified lines were converted to all high level operation with the arrival of the M1A cars, the ACMUs were left in their high-level configuration full-time.