That’s a massive drop in first class provision for a route that is first class heavy.
You also can’t take all 10 cars to Exeter, Honiton Crossing Loop signalling is designed for 9x159 and can’t accommodate longer (also causes problems with diverted IETs).
it also injects massive costs dragging a much bigger train than necessary to Exeter. Logic for any replacement would probably be 5x23m vehicles with 10 Waterloo to Salisbury and 5 forward to Exeter although that does require some works at Salisbury as 10x23m can only use platform 4, fine for the current odd 10 car service but not for regular working.
Just for clarification I wasn't suggesting that they would be a good replacement, or even a replacement at all, rather that it would be possible to get closer to the existing passenger capacity than is generally envisaged.
However even if they weren't too be used there could be measures to facilitate their use:
- mix of 5/10 coach units with the fixed 10 coach units being used on peak hour services and the 5+5 trains being used away from the peak services
- measures to improve Honiton, especially if that allows further frequency of services between Exeter and Honiton, and is probably something which would be needed within a decade or two anyway (I'm reality within 5-15 years from when 22x's could enter service anyway)
- a slightly different mix of how the first class is provided (possibly first two coaches at one end)
- tighter seat spacing, the 159's have fairly poor legroom compared to the 22x's (mostly due to thicker seat construction, not necessarily padding thickness)
Having said that a bimodal 100/110mph unit with the ability to walk between units would be more likely, especially as a replacement for the 442's as well, would work better.
Such an order would probably have the ability for a follow on order option to cater for post Crossrail 2 service, probably the same number as the existing 444's to cater for those extra services.