the 175s, they'd really be used to logically fix the IET issue on Penzance to Cardiff Central.
That, or use the 175s on Cardiff-Portsmouth in 5-car formations as the first step of a cascade which ultimately puts 158s onto Penzance-Cardiff in place of 80x.
Point to note, not all diagrams on Cardiff - Plymouth - Penzance will need to be 5 car for the entire length of the route or for the entire day. Units can be attached/detached to best match demand
Which would make the 158s better-suited for that route (with the 175s doing Cardiff-Portsmouth, which I assume would benefit from being 5-car throughout), since their unit-end-gangways are particularly helpful (and, in my view, should be mandatory) for services which divide on-route.
How many coaches are needed on Okehampton and Barnstaple services ?
Also, how are the platform lengths ?
175’s don’t have ASDO but when I’m multiple do have ‘TUDI’ so the conductor can make it so that only the front units doors open and not the rear sets.
Locking out an entire unit for certain stations (meaning passengers are unable to alight from those coaches) is similar to portion working in that end-ganways really should be provided in my view (to ensure that passengers can move coaches to be able to alight at their destination) in such suituations.
While there are no doubt too many IETs trundling around on local services in the west (not all of which really require 5 cars), with no Bristols via Badminton (and IETs to Bedwyn) there is, in theory at least, a surplus of the things for what they were actually ordered for.
There's certainly a surplus of 5-car 80x (and 80x driving vehicles) but not enough 9-car 80x (just 35 full-length InterCity trains now compared to over 50 IC125s before IEP I believe).
Cardiff Central to Penzance services still utilise the bimode capabilities unlike some strange IET local services within Cornwall. Those locals should be the ones replaced by 175s to help capacity, not the long distance IETs.
Since it's a very small proportion of the route, do GWR even bother to use electric mode on those services? At least they call at Patchway, which in theory allows them to run in electric mode between there and Cardiff (I'm guessing they can't raise/lower the pantograph on-the-move or, if they can, that it needs to be specially set up with balises etc. to tell the train when to do so and that this won't have been done given they don't really want to be using them on this route).
175s will help, but doesn't directly fix problem where some 5car trains are very busy, and nothing 6 or 7 car is available.
Other than the InterCity fleet there are no 5-car units either - you could always make a 6-car or 7-car train by coupling shorter units together, just as you can make a 5-car train by coupling a 2-car unit to a 3-car one. Just need to have enough units (and, where short platforms and/or portion working is involved, for those units to have end-gangways).