The multiple working system on the HSTs is totally different to that on the DVTs. The HSTs use a traditional multiple wire system, with wires or conbinations of wires energise for each control signal, wheras the DVTs use a TDM system which uses one wire, but electronically seperates the control signals into different time slots. Also, whilst the HSTs use special jumper cables between the powercars, so can only work with HST trailers, the DVTs use the train lighting control jumpers so can work with almost any passenger or parcels stock. Some HST powercars were fitted with TDM though, initially for experimental running with class 86s on the WCML, and later for use with 91s before the Mk4s arrived, with one powercar replaced with an electric loco. Originally they were only fitted so they could control the electric loco like a DVT, but since the HST engine needed to be running for train supply (HSTs use a different system to conventional hauled stock), and it was found that the engines didn't like idling for long periods, they were modified so that the HST would provide power aswell, and when leading the electric loco would control the HST, giving very good performance. In theory, there is no reason why these powercars couldn't be controlled from a DVT, although I don't think it was ever done. Once the Mk4s arrived, the HST were no longer needed, and the TDM equipment was taken out. The powercars that were modified were returned to almost original state, the only difference being the buffers on the front, which were retained (these were fitted so that if the TDM failed, as it did, and often still does, the loco could be run around and coupled to the HST powercar without having to worry about setting the emergency coupling bar up).