Too be honest i think it is stupid that it hasnt been done or the governement havnt announced that they intend to electrify it. It would make better use of the dual voltage class 319's but FGW may not have enough to use as a few of them are supposed to be going to Northern. plus perhaps 2 class 319's could run together to form an 8 carriage train during the peaks which could provide a capacity boost.
The displaced class 166's would be ideal for FGW to use on services to Portsmouth, Taunton etc providing they can be cleared and some of them.
Where did this widely spouted idea of 166s on Cardiff - Portsmouth stem from orriginally? It seems insane to me, a regional express service in need of more coaches and everyone seems to want to cascade units onto it that are the same length and less 'regional expressy' (a turbo has 1/3 2/3 doors and perhaps (I've never been on a Turbo myself) 2+3 seating).
Also, while I think the north downs and Thames Valley branches sound like good candidates for electrification, when I stop and think about it I say no. Why? The difficulties of clearing turbos elsewhere, that's why. The GWML wiring should free up enough 165s to rid the one other area where they are cleared (the lines from Newcastle to Chathill, Ashington (re-opening the line and extending the hourly Morpeth service to Chathill & two places on the Ashington section (one service every 3 hours to each)) and Middlesbrough along with Darlington to Saltburn) of Pacers (Whitby branch, Darlington - Newcastle/Bishop Auckland and Newcastle - Carlisle would be all 156s). There would be enough 165s for frequency improvments from Sunderland to Middlesbrough and Darlington - Saltburn too if desired.
That leaves any further Turbos replaced with nowhere to go (except perhaps Chiltern, to displace any stock they have that is within normal loading guage) without spending money clearing lines, money that could otherwise be spent wiring elsewhere. If anywhere were to be cleared for 165/166 operation, I'd suggest Bristol Temple Meads to Taunton (taking over that aspect of the Cardiff - Taunton service, the other end being electrified) and perhaps the Severn Beach line.
Since the North Downs Line is largely in SWT territory the line should be transferred to SWT upon electrification. I assume that either 450s or 458s could be used on North Downs services, although I don't know how many would be required to work the line, or how they would be made available. The most likely scenario would be if 12-14 380s were ordered, allowing some of the Hounslow Loop, Weybridge and Windsor Line services to be increased to 12 cars during the rush hour. 450s would be also be used on Reading line diagrams allowing for a few 458s to be transferred to the North Downs line.
Here's an idea for South West Trains electrification, not sure if it'd release enough stock for North-Downs electrification too though:
- Terminate the Salisbury to Romsey via Southampton Central service at Southampton Central.
- Electrify the line from Eastleigh to Romsey
- Order more 444s (or an updated version, possibly in 4-car rather than 5-car sets) and use them to displace 450s from Portsmouth - Waterloo (and any other fast services passengers complain about 450s on)
- Use one of the released 450s on the Lymington service
- Extend the Portsmouth and Southsea to Southampton Central service to Romsey via the newly electrified Eastleigh route, using the released 450s
That probablly won't shift many 158s, maybe 2? Still, it would allow extension to 4-car of the two busiest Portsmouth - Cardiff diagrams wouldn't it? Maybe you could also electrify to Salisbury from the London direction and use new 444s coupled to an unpowered set on the Waterloo - Exeter services, with the unpowered set being hauled the rest of the way by a diesel loco (as was done on another NSE route in the past, there could be problems with locos not likeing the route though, wasn't 50s not likeing it the reason they now use 159s?).
Bombardier should definatley be kept in the game somehow, whether it's new airport stock, merseyrail stock or replacement of pacers.
My opinion is replacment of some Pacers in the form of an updated 377 for the ValleyLines (inc. Swansea - Bristol and Swansea - Cheltenham).