The Heart or Wales line never was "Great Western" - it was a limb of the London and North Western Railway built into Great Western territory to tap the potential of South Wales, much of it double tracked and built to main line standards.  How it survived the "Beeching Axe" by passing through six marginal constituncies has become a piece of folklaw, and over the years it was thinned down to the thinnest minimum passenger railway.

There have been some small steps back from that perlilous survival over the years - the loop and second platorm at Knighton has been restored (and was in use when Lisa and I passed through in February for us to pass a track maintenanance train), and the loop at Llandrindon was moved from outside the station to the station itself and trains routinely pass there (indeed they swap crews and pause a while to do so).

The signal box, moved (I think) when the loop was moved, is subject to a dispute between the Town Council and Network Rail over ownership and maintenance

From 2020:
https://www.countytimes.co.uk/news/18516278.signal-box-battle-llandrindod-council/

Recent ... but my social media feed (irritatingly) moved on before I grabbed the URL or a quotation, the community use has closed and the buidling fallen into disrepair with no-one looking after it ...