My main objections to removing the services beyond Swansea are the loss of capacity, reduction in through services to Cardiff, and loss of through services beyond Newport other than up the Marches (which is geographically daft, even if it is operationally sensible) and reduction in service quality (loss of buffet cars and 1st class and and probably a downgrade from intercity-standard stock to very-noisy underfloor-engined units, probably inner-suburban ones (150s)).Or just scrap this token service beyond Swansea? How useful is it really? (other than the usual "putting the town on the map" stuff that politicians trot out)
With that in mind, taking away the Paddington service and replacing it with a diesel LHCS service to Bristol/Portsmouth/London Waterloo formed of 6 mark3s wouldn't be nearly as bad as taking away the London service and leaving ATW to fill the gap with services to Swansea using its class 150 fleet.
As for extending electrification to Carmarthen, unless you continued to run trains from the west through to Cardiff (under the wires unless the SDL isn't wired) you really would kill off any hope of rail ever being competive for journeys between Pembrokeshire and anything east of Swansea (if the time spent changing trains doesn't kill that market too). As for frequency, trains every 90-minutes might be ok for the Pembroke Dock branch, but I think the Milford Haven line should really be hourly.
Platform capacity at Carmarthen shouldn't be a problem. There are three platforms already (I know normally only 2 are used) However, I think the other face of the island platform is used for stabling and I think I saw a DMU pulling in there once when the normal platforms were occupied (one by the steam loco Tornado and it's long train). There is also the bay platform next to the buildings (this bay has track, but I have recently realised that track is not actually connected to the operational lines), which might just be long enough to take a 2-car unit.
I've never manged to use either service myself sadly (I've got one of the Pembroke Dock Intercity services though), but that's the impression I got from looking at the times the Carmarthen services runs at on weekdays. I'd be supprised if ATW could provide enough capacity for the 07:30 ex-Carmarthen, shame there aren't connecting DMUs from the Pembrokeshire branches though.The morning service up to London does provide a useful commuter train and business train to Cardiff and Swansea. But the return working has been pretty empty every time I've caught it from Swansea back to Llanelli. This is because it runs far too late to be a really useful return business service from Cardiff or commuter train from Swansea.
Also, the usage figures for Tenby station far outstrip (considering the population of the town) other stations in the area I think. The London services must have some effect on that (or are vital in providing sufficent capacity for it).
I would hope delays will be less problematic once Gowerton re-double is complete.Apart from that, I cna see some merit in local services from Pembroke Dock, Milford Haven and Fishguard starting and terminating at Carmarthen. It may improve punctuality, given the amount of sinle line sections - if the services could be contained they would be less susceptible to knock ond elays from further afield, and long distance services would be less susceptibel to delays in south west Wales.
For journeys from Fishguard and Milford to Swansea it doesn't look like the time differance is that bad, east of Swansea is where rail loses out (and on the Pembroke Dock branch), but maybe with the Swansea District Line it doesn't have to be that way.After all, it seems that those who are able to will drive anyway, as the line cannot compete on speed, frequency or flexibility with the A40 and M4.
To sum up, I think an enhanced DMU service (including 2-car express services to Cardiff via the SDL using 158s) from Pembs plus long-distance LHCS or IC125 trains replacing Paddington services might be better than trying to extend the wires to Carmarthen*.
* Hang on a sec, here's an idea that might just work:
- Electrify to Carmarthen, not via Swansea but via the SDL
- Upgrade the SDL to higher speeds while you're at it
- Have DMU services from Pembrokeshire to Swansea
- Route Carmarthen - London trains via the SDL, and have Carmarthen - Cardiff EMU services running that way too.
- LHCS trains from Pembroke Dock to Bristol or Waterloo in summer to replace the Paddington - Pembroke Dock trains