The only cheap (?) alternative, as I suggested before, is is BOTH bogies are powered on a motor coach and then one powered bogie could be moved to a former trailer. It appears all the electrical equipment is being replaced EXCEPT for the traction motors. But I dont know if a motor coach currently has two powered bogies or one ?. But there are 130 powered trailers that I did not know about until a few posts up

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https://www.whatdotheyknow.com/requ...lling stock Data Sheet 2nd Edition 1.pdf.html (you'll probably want to click on the original PDF on that page to view the document properly)
It's 4 tractions motors, 1 per axle, on the motor cars. I'm not sure what you'd achieve by swapping a motor bogie onto the trailer, to be honest. The limit is still the number of cabs. Assuming that there's sufficient space for engines, compressors, etc, I'd guess it would be easier to just omit trailers to create 2, 4, or 5 car versions, or perhaps 3M+T for a 4 car. If you start converting trailers into half-power cars, you end up with more cars needing traction control equipment, so probably cheaper and simpler to just stick with the existing arrangement. Also, unless axle load was becoming an issue, 2 power bogies per motor car and a lighter trailer probably gives the best case for keeping weight on top of driving axles.
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It's one thing wanting to save some money from the public purse, but sometimes going for the cheapest option simply isn't the best. I work in the public sector and believe me this is what often happens, only for us then to lament the decisions shortly after. Cheap isn't always best. What I am arguing for is a long term strategy that can offer replacements not only for the Pacers in the next 5-10 years, but also for stock that need to be replaced beyond that right across the country. The D-Train is not this solution. The only solution is to get prepared to negotiate deals with manufactures to put forward designs and costings for new stock that can be used wherever in the country it will be needed. Yes it's going to cost more, but it will mean we have a stock programme that's suitable for decades and not just a few years down the road. And in the long term that will represent better value for the public's money. Investment, not just papering over the cracks is what is needed.
Yes, sometimes the better option long term is not the short term cheapest. I agree with you on that. My issue is mainly that there is often a slightly irrational bias against old stock purely because of the age, and no other reason. The age alone really doesn't matter, if it's properly refurbished or remanufactured, and properly maintained; until it simply becomes too costly to maintain it to a reasonable standard.
The 319s are BREL mark 3s, some of the best rolling stock that we ever produced. If they are given a good refurbishment and are still in good shape as far as corrosion and the like goes, they really should be basically as good as brand new stock until they do become too costly to maintain. If the refurbishment is lacking, on the other hand, people will have every right to complain about it. If you think back in time about the likes of the newly GNER-refurbished HSTs (or any similar high quality refurb), they were every bit as good as a brand new train (and arguably much better than a brand new tilt-profile train), aside from maybe being a bit claggy before the engines were replaced.
The D78s, honestly I'm not sure. The idea and project is interesting, and they could help buy us some time, or help keep some lesser routes going. One possibility is that they might help to reduce the rate that we need to build new DMUs at, allowing more of an ongoing production of new DMUs over time, rather than a short term burst of activity followed by nothing for years.