A good start would be to first upgrade the existing Cardiff-Carmarthen line. It currently takes 2 hours by train, or 1 hour 20 by road. It can actually be done in less than 1hr 20min by road, Google maps is telling me 1hr 13mins.
Nobody I know from Aber goes up from Cardiff by rail via Shrewsbury - they either get the bus all the way, or get the train to Carmarthen and then get the bus from there
Cardiff - Aber by bus = 3 hours.
By car = 2.5 hour
Cardiff - Carmarthen by train = 2 hours
Carmarthen - Aber by train = around 2 hours minimum surely
Cardiff - Aberystwyth by train = 4-5 hours?
That is not a competitive journey time.
Upgrading the Carmarthen-Swansea-Cardiff line would therefore be absolutely essential to making an eventual Cardiff-Aber train even remotely competitive to the bus or car and therefore economically viable, and would deliver benefits much quicker and for less investment than the reopening of the line, if say Cardiff-Carmarthen could be brought down to 1.5 or even 1 hour.
In fact such an improvement would benefit the entire of West Wales.
Have any improvements even been muted? A more logical Swansea-Carmarthen route would be to follow the existing M4 inland, and it always seems a farce for Swansea-Port Talbot trains to go via Neath, why reopen the Swansea Town Tawe-Crossing and have a new coastal line?
See my note (in
red) added to your post. We certainly need some serious improvements for rail services to compete with that (or put a much lower speed limit on the roads). The TrawsLinkCymru group promoting the Carmarthen-Aberystwth project actually do state that they would like to see trains use the Swansea district line to cut the journey time to Cardiff, although in my opinion they are not sufficiently ambious, with a target journey time of 1hr 30mins between Carmarthen and Cardiff if I recall correctly. They don't really focus on that though, whereas I think they should be calling for Carmarthen-Cardiff upgrades as phase 1, Bangor-Caernarfon as phase 2 and possibly even Caernarfon-Porthmadog (not Afon-Wen as they propose) as phase 3 before getting to Carmarthen-Aberystwyth.
My view is that there should be Carmarthen-Cardiff express services calling only at Llanelli and Port Talbot Parkway (possibly a new Morriston Parkway station too, but nothing else). It needs somebody to find a load of additional 158s for the new W&B franchise though, as you can't cut Swansea services so the expresses would need to be additional services.
Track capacity is a bit limited between Cardiff and Bridgend though, with the Maesteg stoppers getting in the way of fast services, and just missing out station calls probably isn't enough by itself to make the rail offering fast enough to beat the car. Thus, if you are building new track for Carmarthen-Cardiff I would suggest a new, more-direct, route between Cardiff and Bridgend for fast services (effectively 4-tracking the section, since stoppers and freight would use the existing route via Pontyclun etc.). That new route would serve a much larger population, and hence have a much stronger case, than a new line following the M4 between the Swansea area and Carmarthen.
The black route (3 lane each way) new M4 south of Newport is desperately needed.
Correction, it desperately needs to be cancelled once and for all. Encouraging behavioural change (read: modal shift from private cars to public transport) requires careful use of 'carrots and sticks'. A second M4 around Newport is a carrot for motorists; exactly the opposite of what we need.
There MAY be an issue with the current M4, which is that incidents in the 2-lanes-each-way tunnels can cause lengthy closures of the M4 in one direction. In that case what we might need is a suitable diversionary route, but that must NOT be attractive as a normal travel route, to avoid generating more traffic. Perhaps one way of doing that would be to modify the tunnels so that if one has to be closed the other can be used with one lane in each direction, meaning the diversionary route doesn't have to take all the M4 traffic.
If Ken Skates were to look on Google Earth, he might like to zoom down on Aberystwyth and try to figure out how a re-built railway from Carmarthen is going to enter the town? Much of the Aberystwyth bit has been built upon. Pull down the yellow man for Streetview and you will see practically brand new houses built on the bed of the former railway. I wonder if the residents are aware that if the line were re-built that their homes might have to be demolished? The only alternative would surely be an expensive tunnel through the hill. So, the WG are going to spend £300,000 on consultants to figure this out?
There is another problem with getting into Aberystwyth, that most pepole seem to be ignoring. Whether you demolish houses to reopen the old route or tunnel through the hill, the railway would still have to cross the road which runs from the Morrisons roundabout into town. That would require a level crossing, and creating those is apparently forbidden (the borders railway in Scotland apparently has none, and the Portishead reopening project has had to site the proposed station in a less-than-ideal location in order to avoid one I believe).
