From 1917, the railways began converting steam hauled car/vans to motor carriages. Conversion involved removing the wooden car body from the old underframe and placing it onto a new traction motor underframe fitted with electric traction bogies and switching equipment, jumper sockets and through electric cabling. The old underframes were 'recycled' for use under APL_/ BPL_ excursion cars
Some carriages had been used at Newport Workshops in conversion experiments as early as 1914. Equipment and methods were trialled with many revisions prior to service.
The driving cabs were fitted with a traction controller for powered operation, fuse and switch gear for auxilliary equipment, pantograph air supply, brake control lever and fold down seat.
The steam hauled carriages used for the conversion to ACM were _AC_ cars. These were cars of First Class accommodation. Some of these cars had been lengthened from standard 45' cars up to 1914 by adding van compartments and extending the underframes with splices.
It seems likely that some of these cars ran as steam hauled cars on motor bogies without traction motors and without electric fittings prior to electric service. The old lengthened underframes now surplus were used in another carriage project; the construction of excursion cars coded APL_ and BPL_
The cars that became ACM's between 1917 and 1923 were:
1 - 4, 8 - 18, 20, 23, 25, 26,
28 - 30, 32, 34, 35, 37, 39 - 41, 43
44, 46, 48, 54, 57, 62 - 66, 68 - 127
From 1921 to 1924, ACM's were recoded to _M__ with the same numbers.
For short haul passenger working, several motor cars were used to haul passenger stock. Numbers were 1, 8, 15, 46, 65 and 78. In November 1925 cars 1, 8, 46 and 78 were recoded back to ACM after running in service as M for a while. Cars 15 and 65 were still running as ACM_. At the end of electrification, the motor cars were detached and replaced by steam locomotives for the remainder of the journey. These trains were referred to as 'E' trains; the column heading used in timetables.
These six ACM cars were recoded to _AM_ in 1931/1933.