1960 Line from Bala northwards to Blaenau Ffestiniog closed to passenger services. https://en.wikipedia.org/wiki/Bala_(New)_railway_station

North Wales was a battleground between the London and North Western Railway with their main line along the coast, and friends of the Great Western Railway - the Cambrian - with their line via Llangollen and Dolgellau.  Between then, towns such as Denbigh, Caernarvon and Blaenau Ffestiniog could be reached by branches from both the north and the south.

Blaenau Ffestiniog remains, alone of the three I mentioned, on the main railway network (though TfW are bustituting it at present due to a train crew shortage), but it lost its service from the south in 1960.  Seeing reports of passenger numbers, it was a pretty dire case of a mass transit (if you could call it that) corridor without the masses.  Can't help wondering if - these days - it would attract the tourists, or be part of a political solution connecting south to north Wales while staying within the principality.

The line is also notable that part of the trackbed is now flooded by waters from a reservoir.

If I had to nominate one disused trackbed to become a cycleway it'd probably be Llyn Celyn–Trawsfynydd–Blaenau Ffestiniog. Superb scenery and almost entirely unobstructed – indeed, north of Trawsfynydd the tracks are of course still in place (and subject to a rather unlikely heritage proposal).

In September I stayed in the Plas yn Dre hotel in Bala. In the stairwell was a framed original plan of the diversion planned to take the railway out of the way of Llyn Celyn. As Wikipedia says, it was of course never constructed:

I think the current aspiration for Trawsfynydd is for a "small modular reactor". Don't know whether they'd typically be rail-served.

Here's the heritage project's website:
https://www.balaffest.org.uk/main.php?page=index

and their "State of the Railway" report:
https://www.balaffest.org.uk/newsletters/sor2021.pdf