The Mk3 DVT's replaced the use of the Mk1 BG (full brake) coaches on west coast services . The BG's had an original design speed of 90mph but this was increased to 100mph when the bogie's where changed from BR1's to either commonwealth or B4 types .
In the early 80's certain trains on the West coast using full Mk3 sets had there speed increased to 110mph (using class 87's) which created a problem as there where no loco hauled Mk3 brake coaches at that time , So special dispensation was given to allowing certain Mk1 BG's (NHA's) to run at 110 mph . The only downside to this was that the B4 bogie's had to be put through an increased maintenance cycle with it's added additional costs .
At the same time BR was still running it's Red Star parcels business and the Royal mail was still using ordinary passenger trains to carry sacks of Mail , so the use of a full van with the added security of the Train guard travelling in it was still required .
There was at the time also a problem with rules not allowing the carriage of passengers in a leading vehicle which is designed to run at speeds over 100mph (This has been relaxed since then) . This also meant that the MK4 DVT's had to be non passenger due to there maximum design speed of 140 mph (225 Kph) .
