We have plenty of short DMUs, the need is for more longer trains to cater for ever growing passenger numbers.
So I would go for a 6 car battery/diesel hybrid with two relatively powerful engines of sufficient HP to just reach 100MPH on the level. Additional power for acceleration and to ascend inclines would come from the battery.
Automatic controls would shutdown one engine when possible.
6 cars would be a considerable improvement over the many 3,4, and 5 car DMUs in use at present. Two units could run in multiple to give a 12 car train, up to four units should be capable of multiple operation for ECS moves and rescue purposes, and perhaps exceptionally in passenger use, but not regularly due to platform length limitations.
On a 12 car train, one engine only should be able to provide hotel power for the whole train when stopped at a terminus.
Reliable air conditioning, with windows that can be opened in case of AC failure but are normally locked.
Large windows that align with seats.
Non stinking, reliable toilets with ample water tank and retention tank capacity.
Most seats to face across tables, a minority of unidirectional seats. 2+1 in first, 2+2 in steerage.
Properly sprung or upholstered seats.
A buffet AND a trolley.
Luggage space in overhead racks and between the seats, with careful design the unidirectional seats AND the table seats would line up with windows, the space saved by the unidirectional seating being used for luggage.
And finally two unique features that would cost almost nothing and greatly improve customer satisfaction.
Firstly design the heating and air conditioning such that one side of each coach is warmer than the other.
For winter use install heaters at low level along one side only of the coach, the heat will of course spread out, but one side of the coach will be noticeably warmer than the other.
For summer use supply cold air from ceiling outlets along one side only of the coach (or centrally located air outlets that direct the cool air to one side) The cooling effect will of course spread out, but one side of the coach would be noticeably cooler.
To make it clear how this works, I would make the seats different colours in the warmer and cooler sides of the coach and supply explanatory notices.
"dear customer, in order to best cater for differing preferences, we have designed these trains such that one side of each carriage will normally be warmer than the other. The warmer area has orange seats, the cooler area has green seats" Regular travellers would of course know which they preferred in advance.
Secondly, I would make the lighting level much higher along one side of the coach, customers could choose the dim or bright side according to choice. Far fewer complaints about it being too dim to read by, or too bright to relax under should result.
Asymmetric lighting should cost nothing, and "one sided" heating and cooling should actually be cheaper than the present arrangement.
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