I understand that the speed limit on the SDL is 60mph but that it has decent tracks and therefore fail to understand why higher speeds are not allowed.
Probably because so few passenger trains use it (there are only about 3 trains per day total between Carmarthen and Cardiff that don't call at Swansea and some of those run via the single track behind Landore depot rather than using the SDL). The freight trains, sometimes hauled by class 60s, don't particularly need high linespeeds so I guess Network Rail are only maintaining the track to 60mph standards; cheaper than maintaining it for higher speeds. There may also be some limits due to sighting of user worked crossings etc. Unfortunately, as well as the new Wales & Borders franchisee apparently not being interested in South West Wales, the GWML west of Cardiff is only double track. With the stopping trains to Maesteg (possibly going up to 2tph), Swanline services (hopefully going up to full hourly) and freight, we fitting extra fast services in could be difficult. In my view there needs to be either a new, shorter, route between Cardiff and Bridgend for fast services or there needs to be four-tracking of all or part of the current GWML between Cardiff and Bridgend. If this is only a partial 4-tracking, then it must include at least one station (perhaps the new one at Miskin) to allow fast trains to overtake while the stoppers call. If a path can be found; I would like the new franchise to drop its commitment for 100% fleet replacement in order to retain either the 158s or the 175s (which is tricky as there are pros and cons to both options) for Cardiff-Carmarthen-Milford/Fishguard fasts, with the units also being used on HOWL services and perhaps the Conwy Valley.
Regarding west of Carmarthen: the problem is that we have three branches = Tenby & Pembroke Dock; Milford Haven; Fishguard. The first two only have one train every other hour and the Welsh Government have not requested that this be increased by the new company who will take over from ATW. I think that this is regrettable. So, it would surely be ideal to increase the frequencies on these branches and that would require short trains - certainly running west of Carmarthen. Such trains could also be used as stoppers going into Swansea - as they do at present.
I agree; I think there should be 3tph to Carmarthen (1tph express Cardiff, Port Talbot, Llanelli only and 2tph Swansea-Carmarthen (1tph all stops, the other ommiting Kidwelly and Ferryside)). The Milford Haven line defininately needs to go up to 1tph; not 100% clockface though as it should be a mix of Swansea and Cardiff trains, as should the Fishguard line (to AND from Cardiff for both boat trains, Swansea the rest of the time). Fishguard should have a mix of 2hr and 3hr gaps between trains (basically have the 07:50, 09:50, 18:50 and 20:50 services with a maximum of 3hr gaps the rest of the time).
Unfortunately even a train every half hour Swansea-Carmarthen with most trains continuing west is not possible without more double track (or passing loops) west of Carmarthen. The loop at Tenby is in the wrong place compared to the Fishguard and Milford lines; if a train leaves Whitland for Pembroke Dock half an hour after one to Fishguard or Milford the two trains would get back to Whitland within a few minutes of each other. To allow for a regular half-hourly service between Swansea and Carmarthen there needs to be a dynamic loop somewhere between Whitland and Tenby (probably just north of Kilgetty but I may be remembering incorrectly), and that still only gets you a train every two hours on the Pembroke & Tenby branch (you might be able to go hourly to Tenby, but would have to waste stock sitting in Tenby for hours or replace Tenby - Pembroke Dock with a bus). Hourly to Pembroke Dock I think requires two additional passing loops (the aforementioned one and one west of Tenby).
Also this is perhaps off topic since short DMUs (156s might have been nice, for the Swansea trains) are appropriate for Pembrokeshire services, not expensive 125mph GWR bi-modes. Perhaps if any of GWR's London-Cardiff trains are booked for 5-car sets they could be extended to Carmarthen via the SDL (a 9-car won't fit in Carmarthen apparently).
