I suppose at its simplest we could consider the number of pre and post-privatisation DMUs in operation. There are probably a couple of misnomers in these totals, but to give a good sort of idea of the numbers that we are talking about:
Pre-privatisation DMUs: 754 trains (1545 carriages)
Post-privatisation DMUs: 300 trains (861 carriages)
Diesel Electric Multiple Units (Classes 220, 221 & 222): 105 trains (495 carriages)
Good stats - thanks.
I think that, if CP6 electrification follows on from CP5 electrification (e.g. after the MML is wired then that makes routes like Belper - Matlock, Sheffield - Moorthorpe/Doncaster, Erewash, Derby - Birmingham, Hope Valley more attractive) then those three hundred trains may be enough to run most of the remaining unelectrified services/lines in 2024.
Sounds fanciful right now, I appreciate, but after electrifying roughly a thousand miles of main lines (GWML, MML, Transpennine Main Line) in CP5 then a similar thousandish miles in CP6 could fill in a lot of gaps.
If you believe in the above then you're going to find it hard to justify building lots of DMUs in the short term (since, lets face it, any DMU commissioned today would take around three years to build, we're talking 2017/2018 before we saw any new DMUs in service), since they are going potentially to be surplus to requirements in around ten years time (giving them a lifespan of maybe seven years - nothing like long enough to warrant investing in).
Also, bear in mind that almost all DMU lines are heavily subsidised, so there's little incentive for a TOC to want to speculatively build some "173s".
Whilst there is no doubt a short term lack of capacity in terms of DMUs available I do think that over the next 5 years the situation will improve year on year as electrification schemes are completed and cascades happen
I think that you are right. We may have to chop/change a few routes to suit the electrification map, we may have a tight squeeze toward the end of this decade (when large numbers of DMUs need modification to bring them up to accessibility standards, which might mean we struggle for stock), we might have to get some gaffer tape out to eke a couple more years out of some 1980s stock, we might need a handful of rakes of loco hauled for a couple of years, but I think we'll be okay, we'll not need to build large numbers of DMUs.
Any "scenic" DMU order is going to be so small/ tokenistic that I can't see it affecting the overall figures very much.
Then, by 2024, bi-mode should be proven enough technology on UK trains so that we'll never need to build diesel-only trains.