The electrification project of 1914-1921 included train sets that operated with driving trailers. About twenty trailers were originally planned with codes ACPD and BCPD. The 1921 recoding made them simply D.
In the 1960's, a block of Tait _T__ cars were converted to more D cars. This made the number group 201 to 265. The T cars that were converted were built with driver/guards compartments which were never used as such; they were fitted with seats for passenger traffic. These vans with passenger seats were called "Ringers" because a while circle was painted on the van door. The white circle was an indication to crews that the van was not fitted with train emergency equipment, ie handbrake or emergency brake tap. As an aside, "Ringers" date from 1912 when a group of swing door car/vans entered service as "Ringer" cars.
In the mid 1970's some D and _M__ motor cars were fitted with communication doors for access on two car trains. At about the same time a project was started to save on glass replacement cost. Alternate windows were sheeted up reducing glass area per car by about a third.