In due course the company connected the two sections and reached Dowlais and Merthyr, but had to concede sharing a route with the powerful London and North Western Railway.

The B&MR was always short of money, and was notable for its prodigious gradients, but it survived until the grouping of 1923, when it became part of the Great Western Railway.

Goods and mineral operation also lost its market, and as of 2020, only a short stub to a quarry at Machen remains rail connected.

The difficult terrain southwards would require crossing the ridge of the Brecon Beacons into the high ground at the head of the South Wales Valleys, and the first turnpike connected eastward to Abergavenny and south-westward to Swansea.

It connected the industrial site of Brinore, near Trevil, north of Tredegar, to Talybont-on-Usk, on the Brecknock and Abergavenny Canal.

[5][page needed] The first sod was turned on 18 January 1860; the contractors were David Davies and Thomas Savin, and a lease arrangement was concluded with them for ten years in which they guaranteed the proprietors 5% interest on capital.

It was possible to run a locomotive through to Brecon on 1 January 1863, and on 12 February Captain Rich of the Board of Trade carried out an inspection for approval of passenger operation.

[11][page needed] Passenger stations on the B&MR section of this route were now Rhymney, New Tredegar, Bargoed, Pengam, Maesycwmmer, Bedwas, Machen, Church Road, Rhiwderin, and Bassaleg.

Suddenly the B&MR was unable to deal with its own financial commitments and it starting to miss debenture payments, and the Company went into administration.

[5][page needed] Finishing the line to Merthyr was the obvious priority, and Alexander Sutherland was commissioned to find a means to do so.

Trains of the Hereford, Hay and Brecon Railway, worked by the Mid Wales Railway, started working in to Brecon from 19 September 1864 An east loop was built at Talyllyn, forming a triangular junction there, enabling goods trains from Merthyr to the north to run direct; this opened on 21 September 1864.

It needed a company to work its trains for it, and after a false start the Midland Railway performed this task, from 1 October 1869.

The Midland Railway had a presence at Hereford using running powers, and had long sought an entry into South Wales.

When it negotiated an arrangement to run its trains over the Neath and Brecon Railway from 1877, it had achieved its goal of reaching Swansea.

This brought a considerable volume of additional traffic to the route, and proved lucrative for the Brecon and Merthyr, whose line formed part of the chain from Hereford to Swansea.

[5][page needed] Nevertheless, Barrie observes: As late as July 1869… the Neath and Brecon Engineer, admitted to a House of Lords select committee that if any passengers for east of Brecon arrived by train at Mount Street their tickets were collected, they were put on the engine, and run forward on the high level line to a point opposite and above the Watton station.

(There was also a loop line direct from the Free Street extension into Watton Yard, but the Neath & Brecon witnesses claimed that the B&M would not let them use it, but kept the points locked.

In 1882 the B&MR decided to double the line between Bassaleg, Machen and Abertysswg in anticipation of considerable extra business on the route.

The PC&NR[note 1] was to use the B&MR from Machen to Bassaleg, but needed to pass over a mile or so of the Monmouthshire Western Valley line to reach Newport Docks, and the GWR did not make the facility available at first, on 7 July 1884.

Waterloo and Fountain Bridge platforms for the railmotor stopping trains, were on the separated single lines and accordingly had a service in one direction only respectively.

The first section of line opened in 1888 and in the following year coal was hauled from the Rhondda Valley, and the Company moved on to providing a connection from the Rhymney Railway at Penrhos, west of Caerphilly.

This was a significant civil engineering undertaking, crossing the Rhymney Valley on a large structure known as Llanbradach viaduct.

[14][page needed] Many of the ironworks of the Merthyr and Dowlais areas were declining steeply or indeed closing in this period, and the B&MR network's dependency on those industries made it extremely vulnerable.

There remained a Caerphilly – Machen – Newport unadvertised service for workmen as the only passenger operation on the B&MR network; that too closed on 1 July 1963.

[5][page needed] Goods and mineral traffic between Deri Junction and Pant was withdrawn on 5 August 1963, followed by that between Brecon and Merthyr, and also Pontsticill and Dowlais Central, on 4 May 1964.

[15] Whilst the B&MR was absorbed by the GWR on 1 July 1922, its locomotives were not taken into stock until the four-week period that ended on 8 October that year.

[29] Brecon locomotive depot was shared with the Cambrian Railways, it opened in 1863 on the southern side of Watton station.

Although overhaul of the whole rolling stock fleet was carried out there, new construction was confined to wagons, including the three "market vans".

[5][page needed] Following the death of Dunbar, the post remained vacant until the GWR absorbed the B&MR locomotive stock on 1 July 1922.

[39] Exiting from south-east portal of the tunnel, the line descended for 7 miles (11 km) along a 1:38 (2.6%) gradient by the side of Glyn Collwyn and the (later: 1931) Talybont reservoir.