The main difference between the HST vehicles and the loco-hauled Mark 3A relates to electrical supply arrangements. HST coaches take an industrial voltage/frequency 3-phase supply directly from an auxiliary alternator in the power car to supply on-board equipment such as air conditioning; loco-hauled vehicles take a standard 1000 V DC or single-phase AC train heat supply from the locomotive and convert it through motor generator units under the floor. These convert the train supply to 3-phase 415/240 V 50 Hz AC to power air conditioning and other ancillaries. The two types are non-interconnectable in service conditions. The other main difference is the lack of buffers on HST coaches.