Would it be possible to fit smaller engines in the driving vehicles to increase power? I'm not sure how effective (if at all) this would be though.
Alternatively, is it possible to move the pantograph to the middle of the train, resulting in just one coach with transformers and no engine (as the two end coaches could then be fitted with engines)?
Or perhaps put a medium-sized engine (perhaps 1500hp?) above the floor in each driving coach, taking up maybe 2/5 of the useable length (including the cab) as well as underfloor engines? You could call these end coaches "power cars".
(But seriously, is this at all a viable idea?)
Or perhaps a single vehicle dedicated to housing all the engine power? It would have a single cab to allow more engine space, with the engine running an alternator to power the traction motors throughout the train (an EMU that just happens to get its electricity from a big engine).
Then again, maybe just electrify the route!