Personally, I think there might be something to be said for a mix of 2-car and 3-car units. That gives you flexiblity to run 2-car, 3-car, 4-car, 5-car and 6-car formations as required. One possible drawback though is the need for accessible toilets (assuming there are toilets at all) after 2020. That might make the case for 2-car units weaker.
Are Hitachi capable of building locomotives electric or diesel? I'm positive they are,but are there many examples in Japan for instance?
I believe this has been covered in the IEP debate, Hitachi don't do locomotives. There seem to be a good selection of locomotive options available anyway, Vollosh (class 68 and 88 I think) and Bombadier (TraXX) have already been offered in UK variants. Not sure about Siemens Vectron and Alstom Prima, are UK versions of these on offer too?
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Well the recent locomotive orders have had the electrical connections to run passenger coachs, whether there will ever again be any coaching stock ordered is a good question.
There was an article in Modern Railways quite a while ago about the mrk3s on the London-Norwich route. One option was to build new vehicles to replace the DVTs, but using the bogies from the DVTs. It was commented that such a new vehicle might become the basis for a new mark5 coach design.
However, since the majority of the IC125 replacment will be IEPs I think we are unlikely to see a large fleet of coaches built, maybe a small fleet to replace the non-IC125 mrk3s and later the mrk4s (assuming the mrk4s don't replace the mrk3s anyway).