If new DMUs▸ are to be built, then I feel that serious consideration should be given to hybrid drive (diesel/battery).
In order to give reasonable performance, a diesel train typicly needs from about 300 up to over 500 HP per vehicle.
This much power is required in order to give good acceleration and to ascend inclines at an aceptable speed.
The average power needed is much less, but the heavy and expensive engines need to be sized for the peak power required, not the average.
Instead, one could build a 4 car hybrid multiple unit with say 350HP engines under 2 coaches only, with electric transmission as on voyagers.
The other 2 vehicles would be fitted with traction motors of say 400HP short term rating and suitable batteries.
That would give 1,500 HP for a 4 car set and ensure ample acceleration. The electric motors would only be required when accelerrating away from stops and ascending gradients. The 700 HP from the diesels being sufficient to maintain line speed on level track, and leave a modest surplus for battery charging.
Whenever the train is stationery or coasting, allmost all the engine power would be available for battery charging.
When braking, regenerative braking would be easily achieved, saving fuel and brake wear.
Only 2 engines need be purchased and maintained, and these 2 engines would run for longer periods at their most efficient power output.
On routes with many stops and lower speeds it might be possible to shut down one engine, since a single engine would charge the batteries during the many stops.
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