In what way is halving the XC main line service and ignoring the XC Cardiff service (which is, in case you forgot, half of the passenger service running via Lydney and would have to remain diesel-worked along with all the freight in your fantasy, so hardly a plus for wiring) and missing out the hourly FGW Gloucester-Bristol service "almost correct?
It was almost correct in terms of passenger services which would benifit from electrification of the Bristol - Birmingham and Severn Tunnel Junction - Swindon via Gloucester routes, I only missed 0.5tph from Gloucester to Bristol. For the Nottingham service to benifit you would have to add Birmingham - Derby to the electrification programme too. And as for the other Voyager service, that wouldn't benifit without alot more electrification.
The point I keep trying to make is that an hourly London train (which doesn't yet exist, of course) and an hourly Cardiff stopper constitute a small part of the services operating in the Gloucester/Cheltenham area, therefore, in themselves, will never, ever, justify the expense of electrification, which only makes sense if you can convert the overwhelming majority of the services, including freight.
So of course you need a wider electrification scheme, which means, as I keep saying, the XC route between Birmingham and Bristol. Once that is in hand, then other routes come into play - such as Swindon-Standish junction and Gloucester-Severn Tunnel Junction - not the other way round.
Why can't it be the other way arround? Leaving aside the frieght, which I know very little about (would their orgins and destinations all be on the electrified network anyway?) I would say the hourly Paddington service has the best case for electrification in terms of train length saved per mile of electrification. That is followed closely by XC's Manchester - Bristol service, the case for which would be strengthened by Swindon - Cheltenham being wired. Severn Tunnel Junction - Gloucester then gives 1tph Cardiff - Cheltenham and a possible 1tph Cardiff - Chepstow, strengthing the case for the Derby - Birmingham bit to be done for the Nottinghams. Only then might completing the Plymouth - Edinburgh route start looking achievable.
I don't believe for a minute that as currently set up 16x stock is suitable for the likes of Cardiff-Portsmouth but to say, as you do, that the two-car 175s are too small for most regional express services in one breath but then say that they shouldn't do jobs like the Cardiff-Cheltenham route, to which they seem well suited, with a handful of intermediate stops and an end to end journey time of 80 minutes, is silly. You'd soon want a 175 if you had to spend 80 minutes in a 16x seat on a regular basis.
Putting 2-car 175s on Cheltenham - Cardiff would be ok for the route, but what about spending 135 to 162 minutes from Pembrokeshire to Cardiff on a class 150? If I had a 150/165 (suburban units) and a 175 (regional express) I'd sooner put the 175 on the 135 minute service and the suburban unit on the 80 minute service than the other way arround.
And the Cheltenham - Cardiff, while you say it has few stops, is the line's all-stations service, the other service on the route (XC's Nottinghams) has fewer stops. Even my proposed Carmarthen - Cardiff service, 85 minutes, would have fewer stops (just 3 intermediate stations, Llanelli, Port Talbot Parkway and either a new station (Morriston) or Bridgend).
The proposition that through trains between Cardiff and West Wales should stop operating, with everyone being turfed out at Swansea, is simply a non-starter politically - every AM and MP would be up in arms - so you will need 90/100mph-capable stock, whatever way it passes through the Swansea area.
So, are you proposing that trains from south west Wales to Swansea should continue running onto to Cardiff causing some Swansea - Cardiff services to be 2-car DMUs? If so, I take it you are proposing using 158s and 175s on those? Trains between Swansea and Cardiff need more than 2 coaches, west of Swansea they probably don't need more than 2 coaches very often (the exceptions are Tenby in summer, some Fishguard ferry connections, special events and the commuting peaks).
As I see it, the journey time of over 1hr 40min between Carmarthen and Cardiff running via Swansea is such that anyone using it has a good reason for using the train and will not be put off if they had to change at Swansea. If you are going to provide through trains between Carmarthen (and points west) and Cardiff at all then, in my opinion, they should do it properly and provide a time-competitive service. That means avoiding Swansea. Anything going into Swansea should terminate there (with the possible exception of the 07:30 Carmarthen - PAD and the FirstGW Pembroke Dock services, for peak capacity reasons only).
ATW already has plenty of stock fitting the bill for either scenario on its books, so there is no point adding yet another type to the fleet in the shape of 156s, which are going to be in demand to replace 14xs in the North of England anyway, should any become available from Scotland. And given suggestions that Edinburgh-Glasgow electrification is likely to slip back from 2016
http://www.scotsman.com/news/transport/edinburgh-glasgow-rail-link-faces-three-year-delay-1-2868665 don't bank on that happening any time soon.
Plenty of stock fitting the bill? Then how come the Fishguard boat train (2hrs 30mins with just three or four intermediate stops) is booked for an inner-suburban class 150? The fact is there are very few ATW routes outside the valley lines (which will require EMUs in future) for which a 150 would be suitable.
The original point of this thread was that electrification offered an opportunity to simplify allocations of dmus, to get away from the current situation with operators having penny packets of various types, yet you want to introduce yet another to Wales.
I am not trying to introduce another type. If the 156s came it would be as a replacment for Pacers, 153s and 150s, not in addition to these. Alternatively, we could have refurbished 155s (with improved legroom) instead of 156s, replacing the 150s and Pacers (153s would become 155s). In other words, my proposed Wales & Borders fleet in 2020, assuming no further electrification would be:
- Class 175s (Manchester - Cardiff (3-car), Cardiff - Chester (mostly 2-car, probably), Manchester - north Wales (3-car and/or 4-car throughout)
- Class 158s (Cambrian - Birmingham, Birmingham - Chester, Cardiff - Carmarthen/Pembs, plus perhaps some north Wales services)
- Class 156s or Class 155s (reformed 153s) (west of Swansea, HOWL and perhaps Conwy Valley)
- Class 377s (outer-suburban EMUs) (Swanline, Ebbw Vale, Maesteg)
- Inner-suburban EMUs (inner valley lines)
That's only 3 classes of DMU (compared to 6 today) although the 2 classes of EMU replace the 2 classes of Pacer to make a total of 5 classes.
Wrexham - Bidston would be worked by a class 150 hired in from Northern. Crewe - Shrewsbury would either run through to HOWL or also use a 150 hired in from a neighbouring TOC. The only way I can see that the Wales franchise might need 150s in future is if short-working stoppers were introduced on the north Wales coast (eg. Llandudno - Bangor) to speed up the mainline services. In that case Conwy Valley would be 150 worked and Wrexham - Bidston could be resourced from a pool of 150s in north Wales. They wouldn't be able to get to Canton though, so do Holyhead and/or Chester have the necessary facilities to maintain 150s?