Asking as someone without the detailed geographical knowledge some members show..
Are there obvious inter-operation benefits to electrifying some other suburban routes to Cardiff in terms of electric services being able to continue ?
I don't know whether any through services currently exist between stations known to be electrified under the GWML scheme and stations on the valley lines, or whether the justification for this project will be simply the cheaper nature of electric to diesel operation and / or projected increased revenue from faster services
Further: It's been stated here that GWML freight trains probably won't go over to electric haulage, due to relief lines and links to rail yards not being wired as part of the passenger electrification program. Would wiring the Valleys cover some of the possible end-points and allow 86 and 9x series locomotives to be useful?
I would have thought that the answer would depend very much on the definition of Valley Lines.
My own preference is to use the term to describe the Cardiff Valleys - the lines to Rhymney, Merthyr, Aberdare, Treherbert, Barry Island, Penarth, Cardiff Bay and Coryton. However, the Valley Lines Day Explorer also covers the Vale of Glamorgan line and the Maesteg and Ebbw Vale branches. personally I think that there is less of a priority for the latter three lines, as they have only 1 train an hour compared with 2-4tph on the main sections.
I'm not sure of the current plans for the VoG section, even if the South Wales Main Line is electrified to Swansea it's doubtful that the Vale would be electrified as well as far as I can see.
There are no through services as far as I am aware from stations due to be electrified under the GWML scheme. As to freight, there is very little freight in the Valleys themselves now. As I see it, the business case for Valleys electrification is based on improved acceleration, leading to faster journey times, leading to better frequencies and greater capacity. Hopefully this will then el;ad to better revenue which, allied with lower running costs, will mean lower subsidies in future decades.