Vivarail have sold them in as a low cost short term solution. They are claiming they will cost 1/3rd of the cost of new build to procure (which is a misleading claim as they are comparing new carriages of just under 24m in length with rebuilt carriages just over 18m in length.)
Question is why do we want a low cost short term solution at a time when passenger numbers are higher than they were in the 1930s, electrification is progressing slowly and we have large numbers of diesel carriages built in the 1980s due for withdrawal over the next c.10 years?
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Are they claiming 1/3rd per carriage or per person carried?
What we want is the best solution over the next 10 or 20 or more years bearing in mind, but not limited to, such things as:
* purchase cost, including replacements after x years (as I say with a sensitivity on the x as it appears this could be a key driver of the project economics)
* financing costs (borrowing with some assumptions and sensitivities against future interest rates)
* lead-time to availability (i.e. to generate an income, provide extra capacity, ...) - i.e. if D trains are available 2 years before new build then then get a 2 years extra benefit however it is used
* if capacity available earlier vs new build then this is surely valuable to the travelling public who, assuming the D train is considered acceptable quality, would probably view hanging on an extra year or two to buy brand new as not being their first priority. Indeed if it does get into service erlier then this helps to deal with record passenger numbers quickly which as you rightly highlight is a major issue today.
* maintenance costs of the stock to be used (vs any stock withdrawn if that is to be the case)
* other operating costs
To be clear I am not saying the use of D stock is the right or wrong financial choice over the time period - I haven't a good enough clue about the numbers involved, but neither do I think it's so obviously bad as you seem to make out.
It seems to me that there are people on this thread, Paul Sidorczuk is well known for being one, who just want brand new all, or nearly all, the time so close their minds to the possibility that this could be a viable solution. They look to find all the ways that it could fail (and I think some people would be overjoyed if it did) rather then seeing how / if it could be made to work and if it could for an acceptable cost.
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Look at it another way. In October I'll be in London and I'm planning to use London Underground and South West Trains services (both of which are seen as profitable franchises) but without a local Northern Rail service (seen as a loss making franchise) I wouldn't get to London in the first place! The railways are one big network and if you kill off one bit it can directly harm another bit in another area of the country.
How would the introduction of D trains on suitable routes accross the country 'kill off' that part of the network?