Back in the 1990's when I lived in Bournemouth the XC services where a lot better as they were being run using HST's, although admittedly there was less XC trains running from Bourneouth in those days so the HST's where needed.
To me the most sensible answer to new trains for XC even if Southampton to Reading is changed to OHLE, is to provide XC with IEP Hybrid trains with 5 cars in length. Before anyone says that is too short, bear in mind that the Voyager coaches are 23M and IEP are 26M coaches which as been posted in other threads before you word have the equivalent of a 7 coach 23M train in the 5 coach IEP Hybrid set.
I would then have the 4 car XC Voyagers replace the class 159/158's on the Waterloo - Salisbury - Exter route. The 5 car Voyagers I would pass on to the franchise holder ICWC to make more space available on the Euston - Holyhead service.
The SWT 158/159's would then go on to services to replace the class 142/150 with both FGW & Arriva.
I think you may be misconstruing something I said when people kept saying how wonderful 22xs would be on the Cotswold Line. Agreed, they have pitiful carrying capacity for a 23m coach, set against either Mk3s or Class 180 coaches, but I went on to say that you would need very long formations of 22x vehicles to even match current capacity available on the Cotswold Line, never mind handle likely future growth - the same goes for the XC routes.
If XC was using 180s, I'm confident they would be full pretty much all day now. If they had five-car IEPs, any spare capacity would fill up fast (the proposed 'normal-density' five-car IEP layout is not much of an advance on a 180's capacity), so if you were to go for a five-car IEP, or something similar, you could be perpetuating the 'trains are too small on XC' problem for many more years, when all the forecasts suggest growth will continue, so five cars may not be enough - even with 100 or so more seats than a 221 has.
In this regard, Mark Hopwood spoke the other day about 180s on some eastbound off-peak Cotswold Line services being full before they reach Oxford at present and whether a 5-car IEP was actually going to offer enough capacity for the route in the long run, so the mention of possibly altering IEP formations which appears in the GW franchise invitation to tender could come into play before they even leave the factory.
It is highly doubtful you could assemble a business case for upgrading the SWML above 100mph upon conversion to OHLE.
You have to specify better overhead equipment, likely have to relay the entire line and order new stock from scratch.
We need new EMUs but not enough to jettison a significant fraction of the 444 fleet (there are very few electric routes that would suit them since they aren't conventional commuter trains)
And your basis for saying it would be hard to make a business case is what? I'm hardly an expert but I can see that:
Overhead equipment will be brand new, presumably to the same design as the GWML, which will be suitable for running up to 140mph.
So the track may need work, doesn't mean there aren't ways to do it, or at least get up to 110-115mph.
New stock? The LM 350 fleet has been adapted to run at 110mph without too much pain, so who's to say that their cousins the 444s can't be adapted to run at at least that speed in 25kv mode - and the conversion of the Siemens dc-only sets to work on ac as well is reported to be reasonably straightforward.
