Originally this line was a tramway that was converted to a small wandering single track rural backwater that never made any money, and only linked Hereford to Brecon with no large towns in between.
It was taken over by the Midland Railway and used to access coal mines and industry in the Swansea Valley via the Neath and Brecon line, but was never seriously developed as a through passenger route, unlike the LNW Central Wales line. It closed to passengers, as with all other passenger services to Brecon, in 1962. It joined the Brecon and Merthyr line at Talyllyn junction, which has now been converted into attractive garden, with a display of interesting railway artefacts related to the junction, and is occasionally open to the public.
While this is a beautiful area, the population density is low, and the train frequency was always poor (about 4/day). There are still a handful of buses a day (Stagecoach route 39) between Hereford and Brecon, but there would be minimal demand for a re-opened line, except during the Hay Festival.
It would be preferable if campaigners focussed on reopening lines that might be viable/useful, e.g. Abertillery to Newport to complement the existing Ebbw Vale to Cardiff service.