Trouble is, the railway forgets thr general public is more concerned about getting a seat than whether or not the train has underfloor engines . In an ideal world, if we could start again, build a massive fleet of basic carriages , no engines, just decent seating and have a fleet of locos, electric and bi mode which can haul said carriages about. In summer, more carriages can be added to Cornish and Scottish services, whilst in winter, fewer carriages may be used , and fuel thus saved.
Yes, its a bit simplistic I know, but we are in a mess with the current train fleets and the idea of Loco haulage bad, multiple units good from 20 or so years ago ,has , arguably backfired. Yes TPE isn't a good example, but if carriages had been built on mass, and without being over complicated, then we would be (potentially) in a much better position than.