I assume you are referring to the VR using the term "railcar" after 1945, what's the source of your information?
My 1953 VR General Appendix refers to "Rail Motor" frequently but I cannot find any reference to "Rail Car". There are specific references to "Diesel (Walker) Rail Motor Trains", Walkers were the only source of diesel powered rail motors on the VR at the time (other than the first few DERM conversions).
Ok, firstly to the railcar issue. In the 1965 Northern and Midland Districts WTT, pages 30 and 31 being the Deniliquin-Echuca-Bendigo Passenger Service show the trains coming from Cohuna, Deniliquin and Echuca as Rail Motor (or Rail Motor & Trailer), these all being DERMS, while the service from Balranald is shown as a Diesel Rail Car, this being usually a 102hp Walker. Also the Daylesford service is always shown as 280 H.P. DRC.
However the genesis for the change is shown in my 1947 Northern and Midland Districts WTT which has the introduction of very first Walker on the Wallan - Heathcote service as per WTT 46/48 with hand annotations which has Rail Mtr (AEC Diesel) with AEC with a line through it as does Mtr and Car written above giving Rail Car Diesel.
This then brings us to the second point; the Walkers were not the only diesel powered vehicles at that time. In the mid 1930s AECs 1, 2.9,12 and 15RM were re engine with 4LW Gardners, whilst Double Ended Rail Motor 51RM was fitted with a 6LW in 1937 and then 52RM was also similarly converted in 1939. Thus at the time of the introduction of the Walkers there were 7 diesel powered railmotors in Victoria and had been for over a decade.
As this is getting a bit off topic if you would like any further info please PM me.