I think there is some confusion here, as there must be two AC buses. One takes 25kV from the pantographs (on both end cars, only one raised at a time) to the transformers (in one or both end cars). This must pass through all the intermediate cars with no electrical connection to them. The other takes the power at a suitable voltage from the transformers to the supply converters in electric mode, therefore must be AC not DC. In diesel mode it appears each GU drives its own local supply converter only, so must be disconnected from the lower voltage bus, which also avoids the issue of having to get multiple gensets to feed in phase onto the same bus. There must also be yet another bus, probably on the DC side of the traction converters, which allows one GU to power the entire train's auxiliaries and may also allow traction power to be cross-fed to traction converters on other cars.