Working back to Plymouth as passenger train seems good use of stock.
I agree that stock should be used to best advantage but there is an interesting mathematical trade off between hours in use/miles run and time between failures. When Hull trains had 5 Meridians and used 4 then their
reliability as measure in miles per 5 minute failure was the best of the modern DEMU▸ fleet Roger Ford Modern Railways annual survey). After they dropped one on the floor and wrote it off, and they were down for 4 for 4 the following year the relaibility plummeted.
That's why I'm not sure about about many of what I call stock move diagrams like Great Malvern to Brighton which in my book is really two separate services. Which means if a unit is unavailable you lose two trains rather than one had there been two units on the diagram. One doing Great Malvern Bristol and one Bristol Brighton.
It would only require a few extra units which would have two benefits trains could be stengthened in the peak and a more frequent and reliable service could be run to many stations such a Melksham during the Off Peak.
So we should forget about having extra trains up the Cotswold Line between Oxford and Moreton-in-Marsh from September, should we? These will almost all be provided by rediagramming Turbos that would otherwise be sitting in the sidings at Oxford. I feel pretty confident their reliability will not plummet, despite putting another 60 miles each round trip.
Trying to draw comparisons between maintenance needs on a very small fleet of high-speed rolling stock and a rather large fleet of dmus that have a rather less stressful life is a pretty dubious exercise and I think the depot staff at Bristol might be surprised to learn that if a Malvern-Weymouth train hits trouble, you lose two workings, given the number of times it is obvious from posts on the website about trains being restarted at Bristol that a dmu has been mustered from St Philip's Marsh to pick up a duty when there are problems.
It would only require a few extra units
And when a few extra units finally do turn up from
LM▸ , they will be used to ease overcrowding around Bristol, not allow Cornish branch dmus a rest stop they really don't need.