The Harris trains were introduced to service between 1956 and 1970. Planning for these trains had proceeded from at least 1948. They were a replacement for the swing door trains, the heritage of which went back to 1887.
The trains were introduced with three classes: M for motor cars, _T__ for Trailer cars and _BT_ for Second Class Trailer cars.
The railways used the letter M for all electric suburban cars and used number groups to split the different types.
With the introduction of Comeng built suburban trains in 1981 and extension of contracts to the late 1980's, the Harris M car numbers were going to be duplicated. To avoid confusion, the Harris M cars were renumbered into the 1000's; 501 became 1501, etc.