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The Brecon and Merthyr Junction Railway (B&MJR) was one of several railways that served the industrial areas of South Wales and Monmouthshire. It ranked fifth amongst them in size, although hemmed in by the Taff Vale Railway (TVR) and Great Western Railway (GWR). 
It notably gained the unfortunate nickname of "Breakneck and Murder Railway" owing to a certain tendency towards having accidents - which, owing to the steep gradients, were generally rather severe.
The B&MJR was once described as a "lively octopus in a tank of sharks", but despite the aggressive activities of the "sharks", it survived until the railway grouping. In the process, it absorbed several smaller companies and by negotiating running powers over the lines of other companies, it established links between Newport docks and Brecon, and hence into Mid-Wales.
1858 The company was established by a Bill of 1858, with the directors including several prominent Brecon citizens.
1859 Company was incorporated.
1862 The Beacons tunnel (also known as Torpantau)was completed by 1862, and runs between Brecon and Pant commenced in 1863. The complicated series of amalgamations (including its originator the Hay Railway) can best be appreciated here to explain how the B&MJR came about.
The Southern section linked Bassaleg (where there were connections with the GWR and the London and North Western Railway) and the ironworks town of Rhymney, near the head of the Rhymney valley. The TVR ran up the same valley in parallel with the B&MJR.
The Northern section linked Deri junction by means of running powers over a section of the Rhymney Railway in the Bargoed Rhymney valley to Pant, Pontsticill and Brecon via a tunnel through the Brecon Beacons.
Initially, the only connection to Merthyr Tydfil was by means of a horse-drawn bus from Pant, but by 1868, a connection with Merthyr had been established by sharing lines with Vale of Neath, London and North Western and Taff Vale railways. This involved the building of nearly seven miles of line from Pontsticill to Merthyr, with an almost continuous descent of 1 in 45-50, two complete reversals of direction and the construction of two viaducts to carry the line over the Taf Fechan at Pontsarn, and the Taf Fawr at Cefn Coed. The Pontsarn viaduct is 455 feet long and 92 feet height, whilst the Cefn Coed Viaduct (or Pontycapel) is 770 feet long with a height of 115 feet.
The section to the north of Pant was primarily a passenger service, serving isolated farms and villages. South of Pant, it was mainly a mineral line and carried coal from the mines down to the Newport docks.