In 1961 the _OT_ tank wagon group was reclassed to TW or TWF. This class system allowed for a speed designation. Low speed wagons were coded to TW and high speed tanks classed TWF.
In 1964, tank wagons capable of bogie exchange operation were placed into service coded TWX.
The bogie Oil Tank ( OT ) number group started at 84 and went to the high 500's. Tanks 1 - 83 were fixed wheel. These tanks comprised water tank wagons with different codes and tank wagons for the following products: bitumen, alcohol, diesel, white spirit and petroleum. In this mix, tank wagons were run by different petroleum companies.
Tank wagons were either
As tank wagons were placed into service in limited numbers and by different companies there was a large variety of underframe types and tanks. Then as traffic changed, relettering between TW and TWF occurred.