Also known as Beacons or Beacon Summit Tunnel - Torpantau claimed the record for the highest tunnel on the UK's standard gauge network at 1,313 feet above sea level. Reached by a three-mile climb from the south, trains passed through it curving sharply to the right before emerging onto the notorious Seven Mile Bank at a falling gradient towards Brecon. This was an extremely ambitious attempt at somehow maneuvering long carriages through the winding hills of mid-wales. The tunnel opened in 1863 although construction work had finished the previous year.

The Brecon & Merthyr Junction Railway (B&MJR) which passed through the tunnel gained the unfortunate nickname of the "Breakneck and Murder Railway" due to the number of accidents it suffered which, thanks to those steep inclines, were often severe. Neither was its reputation done any favours by the slowness of its trains. By the time the section through the tunnel closed on 4th May 1964, the line was only carrying freight. Passengers services across the B&MJR system has ended by January 1963.

Torpantau Tunnel itself is 666 yards in length and features a masonry arch springing either off shelves cut high into the rock face or lengths of brick or stone sidewall which were added incrementally over time as the need arose. Refuges are incorporated throughout. The south portal is masonry-built and wedged into the end of a vertically-sided cutting, the west face of which is retained by a wing wall for a short distance.

The north portal (actually ESE-facing) is a larger stone structure, projecting out into the cutting which has more gently-graded sides. Though generally in reasonable condition, some small falls of loose rock have occurred, accumulating on the solum, and a stream flows through the tunnel resulting in both approach cuttings being waterlogged.

Torpantau station was left derelict after the passenger trains ceased operation in 1963 exactly 100 years after it opened, and it wasn't for another 18 years until the narrow-gauge Brecon Mountain Railway occupied the trackbed which lies south of the tunnel where the station once sat, having now served since 1980 the mountain railway asserts the long-term aspiration of extending its operation northwards through the tunnel. Until then, the tunnel lies dormant among the lonely mountains.