I don't think GWR like the idea of Gloucester to Taunton - the idea of Cardiff to Penzance has been discussed in this thread.
GWR might be considering Cardiff-Penzance, but in this speculative topic it has also been suggested that we should "seperate the more local flows" so that we can "provide the longer distance passengers with suitable stock. Extending the GWR Cardiff-Taunton into a Cardiff-Penzance service would have the opposite effect - ie. combining long and short distance passengers on the same train.
But it is not clear to me what market Cardiff to Penzance is intended to serve. It seems to me to be another Jack-of-All-Trades route, like the Cardiff - Solent -South Coast axis. The idea of separating out the greater Bristol commuter radius from longer distance passengers was behind my suggestion of Gloucester - Taunton.
Separating various flows according to their function can then inform the choices made about replacement rolling-stock on the services being discussed.
You seem to be making a similar point to me here; Cardiff-Penzance (assuming it is an extension of the slow GWR stopper via Weston-Super-Mare) just seems to be a crazy hotch-potch. Gloucester - Bristol Temple Meads - Weston-Super-Mare - Taunton makes alot more sense as a service, that would clearly be the local stopper with XC being the fast service on the same route (save for bypassing Weston-Super-Mare and coming from Cheltenham rather than Gloucester - in fact the GWR service should probably start back from Cheltenham for that reason - to provide connections in/out of the XC).
Not sure I would go for ½-hourly local services between Cardiff and Bristol, I would need some convincing that there is a level of demand between, say, Lawrence Hill and Severn Tunnel Junction to support such a frequency. And as for Pilning...
I was suggesting that the Cardiff-Bristol bit of the Cardiff-Taunton be transferred to TfW and doubled to every half hour (with the Cardiff-Portsmouth staying with GWR and possibly losing some stops). The Cardiff-Taunton doesn't call at Lawrence Hill and Pilning anyway (certainly not every hour) so neither would my service (not every train at every stop anyway - with EMU acceleratation there may be scope for 1tph to stop at Lawrence Hill and the other at Stapleton Road to give them a through service to Cardiff) but I think Severn Tunnel Junction and Cardiff Parkway could do with 2tph to Bristol Temple Meads. Stopping the Cardiff-Portsmouth at Cardiff Parkway would damage it's potential as a long distance service, hence why I suggested there needs to be a seperate 2tph local (outer suburban) service between Cardiff and Bristol.
Swansea extensions I would handle with the long-distance trunk services; since these would need some form of onboard traction power supply anyway, the (penny-pinching) lack of electrification should be less of an issue. Indeed, hazy memory tells me that, back in the days of Wales & West, some Pompeys got as far as Carmarthen!
I expect some Portsmouth services even made it to Pembroke Dock / Milford Haven / Fishguard Harbour in Wales & West days, but can't be sure. However, there is a dire need for additional capacity (particularly a facility for fast trains to overtake stoppers) between Cardiff and Bridgend / Port Talbot and competing demands for capacity between mainline and metro services over what is currently a two-track railway. Do you cut the Swansea-Manchester back to Cardiff-Manchester in order to protect the Portsmouth route through to Swansea? Given a choice between 3tph fast between Cardiff and Swansea or 2tph plus 1tph fast to Carmarthen (via Swansea District Line) I would choose the latter, meaning Swansea has to choose between Bristol and Manchester or have the Bristol service be part of the metro. My attempt at a compromise is a Bristol-Swansea semi-fast / outer suburban as part of the metro calling at Neath, Port Talbot, Pyle, Bridgend, Cardiff Central, Cardiff Parkway, etc. But it needs doors at thirds because of the extra stops like Pyle, Cardiff Parkway, Patchway and Stapleton Road; and it needs electrification to Swansea in order to do it with an EMU.
To be honest Cardiff to Taunton has always seemed like a Cardiff to Bristol and Bristol to Taunton service joined together for operational convenience rather than any significant demand between say Newport and WSM.
In which case, joining the Taunton to Bristol bit up to a Bristol to Gloucester/Cheltenham for the operational convenience of not having too much terminate in Temple Meads makes alot of sense. In fact, if the bays in the old train shed (at one point intended for the Paddington - Bristol Parkway - Bristol Temple Meads fast IEP services) are still going to be built they are on the correct side of the layout to be used for terminating services from south Wales.
New class 195s would be just fine.
Don't 195s have ironing board seats, no unit end gangways, only one toilet per unit, poor alignment of seats to windows, huge amounts of standing room and the curse of blight on electrification prospects?
Snow1964 siad: "There is very high passenger churn at some stations like Salisbury and Bath, so decent capacity doors or vestibuleS are needed to avoid station delays. It would be eased if trains were regularly nearer 7cars than 3-5cars."
Wouldn't the least expensive option to be to adjust the timings at busier times/places to anticipate the longer dwell?
To me the best option would seem to be a combination of both; first make sure all trains are at least five coaches during the main part of the day (possibly reducing to 3 or 4 late in the evening when it's quiet), strengthened to 6 or 7 at times if it really needs it AND adjust the timings to anticipate the dwells. To paraphrase somebody else, capacity on regional express routes should be about number of carriages and seats, not compressing people and standing capacity.
It may be that the route could even take 24m or 24.5m carriages which potentially could give another 24-48 seats in 2+2 formation 6car train. I know the 26m bi-modes have used the whole route north of Westbury on diversion so it is only the Portsmouth end where max carriage length might be restrictive.
I accept not that many lines need a cross country tri-mode, but isn’t going to be harmful if a common design was also used to replace the shortened HST castle sets
While there aren't that many routes that need dual-voltage electro-diesels (using that term to avoid confusion with TfW's battery/electric/diesel tri-modes), aren't modern 3rd rail EMUs built to be 25kv AC overhead compatible/convertable? Is there much in the way of technical differences (ie. NOT interior specification differences) between class 450s, 350/1s, 350/2s, 350/3s and 350/4s, or could you (for example, I'm not sure why you would want to) take the third rail shoes off a 450 and put them on a 350/4 with little more than undoing/fastening a few bolts?
If one EMU is much the same as another on a technical level, then you could probably produce a batch of dual-voltage electro-diesels for Cardiff-Portsmouth as part of a much larger order for bi-modes which have only 25kv AC and diesel mode. Due to the need to decarbonise, we should have ended DMU production a few years ago (the Civity DMU family, with the possible exception of the 196s because they are only a small fleet, is an enormous mistake), meaning plenty of need for bi-modes as DMUs fall due for scrap.
If, on the other hand, the third rail requirement necessaitates something rather more bespoke then, on the subject of carriage lengths, I would like to throw 21.8m out there. If you did that, an 11-car formation (either a 5-car plus a 6-car or a 5-car plus two 3-car units) would be a similar length to a 12-car 450 and a bit longer than a 10-car 444 or 158/159. This I feel would be useful for SWR on the Waterloo-Exeter route, provided a 444-style door layout of course not cavenous doors at thirds, with GWR getting some units of the same type for Cardiff-Portsmouth as part of the same order. Throw a few electric-only ones on the order as well for the Portsmouth fasts (giving Southern the 458s to allow them to put something with toilets on the Coastway in place of the basically life-expired 313s) and job's a good'un.