I think we have been round this one before.
I think we probably have; but here's my view (again?).
(Re)building a new railway is an expensive process, both financially and (probably) in terms of greenhouse gas emissions. That capital cost is only justified if the benefits (particularly in terms of greenhouse gas reduction) are significant. In order to acheive a sufficient greenhouse gas reduction such a railway would need to attract significant modal shift from private cars. My gut feeling is that this is not acheivable unless said railway offers a journey time faster than Google maps etc. suggest it would take somebody to complete the journey by car.
The original routes from Carmarthen to Aberystwyth, Merthyr Tydfil to Builth Road, Builth Road to Moat Lane Junction and Afon Wen to Bangor were all indirect compared to the current roads between Carmarthen and Aberystwyth, Merthyr Tydfil and Newtown and Porthmadog and Bangor. While I think there is merit in looking at linking these places directly by rail, I do not support reopening of the original routes which I suspect would not offer sufficiently speedy rail journey times (in the case of Carmarthen - Aberystwyth, the study that has been done confirms this and the solution - building a new tunnel which would also bypass the Gwili railway - was dismissed as too costly).
Of all the options, I think the section most likely to be a success would be Bangor - Caernarfon. The old route from Caernarfon towards Afon Wen might also be suitable as far south as Bryncir but beyond there I think the line needs to head directly towards Porthmadog (which would be a brand new alignment) rather than Afon Wen which is only direct enough if your traffic objective is Pwllheli and not a north-south link to mid Wales (Machynlleth and Newtown).
For though traffic within Wales, I offer you three other options:
1. Builth Road to Moat Lane Junction plus Afon Wen to Bangor
2. Merthyr Tydvil to Moat Lane Junction plus Afon Wen to Bangor
3. Redraw the national boundary to move Hereford and Shrewsbury into Wales
On that last one, you forgot that Chester would also need to be moved into Wales (and Crewe if you want to avoid a reversal).
