What if the DMU is substantially more expensive? Maintaining a six cylinder engine will not be much cheaper, if at all, than a twelve cylinder engine. The first cost will be lower but changing oil, and changing an oil filter will incur much the same cost. The higher speed six cylinder engine will wear faster and require overhauls more often, and require replacement more often. And there are ten engines on a five car Explorer or a five car Velocity, and two on a five car XPT.

The original XPT design included driving control trailers (see John Dunne's history of Comeng) This was because it was being offered instead of DMUs, on the basis that it was cheaper in the long run. If the driving control trailers were designed to have a through corridor connection when coupled (there are many such designs in the UK), a five car XPT fitted with Scharfenberg or Dellner couplers could split into two and three car sets at Werris Creek, just as the Explorer does now.

Diesel engines last longer when operating at near full loads but their fuel consumption is much more consistent through the power range than petrol engines. Manufacturers issue complex diagrams of diesel fuel consumption but the variation in specific fuel consumption (in grams per kW) due to load is relatively small.

The Tasrail locomotives have a single V-12 Caterpillar 3512 engine, generally similar to the XPT Paxman VP185 (the Cat has a 175mm bore). Another company, NREC supplied multi engine locomotives in Australia. Two 1200 class demonstrators that have seen almost no work, even after a move to SA, and seven PB class at Port Kembla. BHP at newcastle used twin engine locomotives from GE from the mid 1950s until the plant closed, so using them at Port Kembla is no surprise.

Incidentally, the not generally liked 1200 class have three engines of the type fitted to Velocity DMUs for propulsion (Cummins QSK19), except that they sit upright rather than on their side under a DMU.

Even in the USA, multi engine locomotives are disliked. I visited Roseville, west of Sacramento where there Union Pacific has a big hump yard with two humps. A pair of single engined GP38-2s was working one hump and a pair of GP39-2s was working the other. One multi engine locomotive, purchased for the humps with government money, was sitting shut down beside the workshops. The multi engine units had lower exhaust emissions and might have used less fuel, but they were more expensive to keep working and were not regarded as reliable. Locomotives with locomotive engines are cheaper to run.

M636C

Perhaps the better option is for
- 3-4 car sets
- Lead car contains drivers cab, large donk, smaller Aux donk, luggage
- Large donk has power to run 4 car train at 160km/hr
- Small aux donk, can run 4 cars, but another 2 cars on reduced demand during emergencies.
- Lead car and adjacent pax trailer have traction motors, assume to be DEL.
- If there is still room, I'd had space for EC seats and/or toilet for use by pax cars.
- 2nd car or adjacent to lead pax car contains FC seats and rear end buffet
- 3rd car contains EC seating and toilet for use by both cars.
- 3rd car contains drivers car for reverse running at line speed but with walk through capability.
- 3rd car can be decoupled within reasonable time frame for insertion of 4th EC pax trailer car.

To make large sets, another one of above is added to driver trailer in mirror image.

Sets can run as 3 car, 4 car or combined with other. 6, 7 or 8 car.

Hows this?

EDIT:
I just realised the Large Donk is in the range of 1600kW, almost back to XPT design and design issues regarding weight on the power car. Spreading the traction motors over 8 axles not 4 like the XPT should help spread out the weight someone, also the traction motors will be smaller and more typical of larger EMU models. RTT is 180kW. So maybe few tonnes shifted off the power car.

Distributed traction should also improve traction/acceleration and I assume train performance over the XPT.

Note CTT has two diesel traction engines per power car, but assume this is due to the hydraulic traction system employed there to save weight. Also the govt only bought 4 locos with the original order so lack of spares.

The Aux genset requirements would be around 300kW, a large genset in its own right. Perhaps better done as 2 x 200kW units. One placed under the driver trailer and the other under the power trailer working together, or duty + boost as demand requires.

With the Genset gone, the power car should now have plenty of room for luggage and even a toilet for the FC car behind. FC car would then only have space lost for the buffet. Both the EC trailer and driver trialer cars would have a toilet placed at the lead power car end so there is always a toilet where pax cars join.

The walk through drivers compartment in the driver trailer I would not be open for passenger use, just staff and even then not designed for use at speed. Ideally the on board work load for the staff would mean each set is pretty much stand alone and staff are uterlised effectively. Again I would still have vending machines to manage most of the cold drinks and snacks to reduce the buffet staffing demand.