Electrification onwards from Cardiff to Swansea is very much a part of Network Rail's long term, green / carbon zero agenda, and I would be very surprised to find a vote of members here to come up with a "don't do it" outcome.
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In writing the above, I am not answering "should Cardiff to Swansea be electrified next?" - I look at Chippenham to Bristol Temple Meads, and Didcot to Oxford as top candiates, with Bristol Temple Meads to Bromsgrove and Basingstoke to Reading as also in my thoughts for the most immediate/first candidates in a rolling program. I am not addressing the question "should we upgrade the line while we are at it?" either.
I will answer "should Cardiff to Swansea be electrified
next?"; not in my opinion. Even looking just at the
GWML▸ , Didcot-Oxford and the routes into Bristol Temple Meads (both via Bath and via Bristol Parkway, assuming the
PAD» -
BRI» via Bristol Parkway services haven't become a casualty of COVID) should come first. A key reason for that is that:
A rush to electrify without doing upgrades first adds significantly to the cost of those upgrades. Someone once told me how much more Temple Meads Roof would if it was electrified first. I can't remember the amount but it was very large! I expect we could say the same about Bristol East Junction. If improvements need doing to raise line speeds between Cardiff and Swansea then they must be done before electrification.
Between Cardiff and Swansea (and, in particular, between Cardiff and Bridgend) there are capacity limitations with a mix of slow (Maesteg and Swanline) services, freight and fast passenger services sharing just two tracks (with a few freight loops). That needs to be addressed before electrification.
May I pose a related question, would it be acceptable to break existing services so that only or predominantly, electric traction was used on this route?
Would changing trains at the limits of the wires be acceptable?
At Cardiff? I doubt it. It would however, I feel, make sense to terminate more services at Swansea. Manchester to Milford Haven is so hopelessly indirect that most who would be put off by needing to change would have been put off by the journey time anyway. Even Cardiff to Carmarthen is absolutely thrashed by the M4.
There might be through journeys from Manchester to Pembroke, both for the ferry and for beachy holidays.
Most of the through trains from Manchester go to Milford Haven not Pembroke Dock anyway - I think it's just one or two evening workings that go to Pembroke Dock. Even then, the ferry terminal is not obvious from the station and probably quite a long walk (I don't even know how to get to it). If you want a ferry to Rosslare Fishguard Harbour is probably a far better bet.