A new design suburban train was issued to service from 1973. They were a replacement for the remaining swing door trains approaching 90 years of age. In the contract, driving trailers were specified. The code was D and the number group was 301 onwards. Apparently one hundred of these D cars were to be built of the 300 cars ordered: 150 x _M__, 50 x _T__ and 100 x D.
By Easter 1974 there were operational problems with the front communication doors. The doors leaked water and were very draughty. Following an industrial dispute, the doors were permananently sealed closed.
The D cars ran in trains sets marshalled M-D-D-M-D-M or M-T-T-M-D-M. The two car M-D sets ran off peak services on the St Albans, Glen Waverley lines as well as the Pakenham shuttle between Dandenenong and Pakenham. This shuttle was the last service to use an M-D set.
In 1978, the impending operation of the underground loop saw a change in the train consists with D cars not required except for fringe suburban working. Sixty eight D cars were delivered, 301 - 368. Numbers 364 - 368 were never fitted out and were placed in traffic as _T__ cars. D cars 301 - 363 were progressively withdrawn from service and modified to T cars. Electrical equipment from the cabs of the stripped cars was re-used in the extended M car contracts at the time.
Initially, the D cabs were removed and the ends rebuilt to T car style. The pace of work was slow and costly. Later D cars were simply issued as built with cab windows painted over, doors locked and driving equipment stripped out.
For a while, some D cars converted to T cars were in train consists still coded D. The reason for this was unusual. The specific T number group was in the 990's and the Workshops did not have enough '9' transfers to complete the code and number change. Cars were recoded about 2-3 months later.