From 1916 to 1918, twenty six ACPM cars were built. In 1917, there were sixteen conversions ex ACP_ and one car in 1921. This was part of the build up for the start of electric traction service in 1919/1920.
The steam suburban service at the time included First and Second Class compartments. Electric service would duplicate this and provide for First and Second Class motor cars and trailers.
Cars built new were ACPM numbers 59 to 84. Cars converted from ACP were: 1, 8, 39, 42, 46 - 58. These cars had been ACP 49, 56, 87, 90, 94 - 106 respectively.
With the change in electric traction codes 1921/1922, the ACPM class was relettered to _M__. To segregate swing door cars from sliding door stock, the sliding door stock was renumbered into the 200's. ACPM 1 became M 201, etc. Swing door cars retained numbers below 200.
The ACPM class was relettered by the end of 1922.