For the Melbourne to Sydney 'Southern Aurora' train service several car carrying wagons were converted for Motorail use. The Motorail service allowed passengers to travel on the train and take their car as well.
The vehicles were coded MBK and were numbered 2951 to 2595. They were converted from NSWR 'BKX' car transports in 1975.
In 1978/1979 the vehicles were fitted with high speed bogies and recoded to MBY. The numbers were retained.