The ABM class were combined First and Second Class motor cars with a driving cab at each end. The original planning of 1916 had ten cars of this type in service. These 'double ended' cars were to be coded ABCM.
All the ABM cars were recoded from _M__ between 1926 and 1937.
One end featured the full guards van compartment and motorman's section with the opposite end having a motorman's section only. This end was to be modified with full featured curved end. Some of the conversions from _M__ stock involved splitting a passenger compartment and adding driving windows and fittings to the 'square end' of the carriage.
The cars were used as single cars on sections where there was light traffic. If traffic increased at 'peak' times, a _D__ car was added.
With the replacement of the swing door fleet by the new 'Harris' trains from 1956, the cars became surplus. Opportunity was taken to use these double ended cars as parcel's coaches, coded _CM_. This was to supplement the parcels coach fleet to cope with additional traffic.
One ABM car was converted to a works sleeper, code _WW_.