[2] Crawshay Bailey was the driving force behind the development of tramroads in the Nantyglo area which served to bring raw materials to and from his various ironworks.

First, in 1822, he opened a 5.5 miles (8.9 km) plateway known as Bailey's Tramroad between the Nantyglo Ironworks and a wharf on the Brecknock and Abergavenny Canal at Govilon.

[4] In February 1859, Crawshay and his partner Thomas Brown acquired the Beaufort Tramroad between Brynmawr and its junction with the Llanvihangel Railway at the canal wharf in Gilwern.

[2] The next step for Bailey was part-conversion of the tramways into a standard gauge line connecting prosperous Merthyr with its reserves of coal and iron ore and Abergavenny which was in decline.

[8] After Govilon the line meandered beneath Gilwern Hill through the Clydach Gorge, climbing upwards on a breathtaking 7 miles (11 km) ascent at gradients of 1 in 38, with the upper section hewn out of a hillside shelf.

[9] Beyond Brynmawr, the line rarely descended below 1,200 feet (370 m) above sea level, crossing despoiled treeless moorland and the heads of the mining valleys to Dowlais.

[11] Work commenced immediately on the line's construction, the contract for the section between Abergavenny to Brynmawr having been let to William McCormick.

[18][17] The first train ran as far as Govilon, the only completed station at that time, and was driven by the wife of Captain James Hill, the company's vice-chairman.

[20] However, the financial state of the B&M eventually led it to conclude two agreements with the L&NWR which granted the larger company favourable terms for access to Dowlais and Merthyr.

[19] At the same time, the L&NWR opened a station at Dowlais Top, close to the summit of the line at 1,250 feet (380 m) above sea level,[20] where a connection was made with the B&M for the exchange of traffic.

[19] A second agreement in 1874 saw the B&M agree that its own Merthyr branch from Morlais to Rhydycar junction become a joint line with the L&NWR, with the larger company taking advantage of the smaller company's financial state and repaying half the costs of construction at £25,000 per mile – far less than it would have needed to pay for access to Merthyr.

[22][24] The link opened on 9 June 1879 and services ran through to Merthyr (High Street) using running powers over 44 chains (890 m) of GWR track from Rhydycar junction.

[29] Until the closure of the line, services were operated by LNWR Webb Coal Tanks, of which some 55 were allocated between the three sheds in 1919, with Abergavenny receiving 37, Tredegar 14 and Blaenavon 4.

[1] The 25-mile (40 km) journey between Abergavenny and Merthyr, including 15 intermediate stops, took 1 hour and 40 minutes, which left it vulnerable to competition from other modes of traffic.

[30] The last public timetabled service ran on Saturday 4 January 1958, the last train running was the 08.30 p.m. Abergavenny Junction to Merthyr station, hauled by GWR 5700 No.

[30] The last passenger-carrying train was a special organised by the Stephenson Locomotive Society on Sunday 5 January 1958 made up of 5 eight-wheelers (a GWR corridor and 4 LMS vestibules) hauled by L&NWR 0-8-0 'Super D' No.

[38] Little remains of the former line save for a section of trackbed in the Clydach Gorge between Brynmawr and Llanfoist has been converted into a long-distance cycleway.

[39] The cutting in which Beaufort station was situated has been infilled and the line from Trevil onwards has been obliterated by the A465 Heads of the Valleys road.