The head of the River Taff valley, at Dowlais, Merthyr Tydfil and Rhymney, was the scene of huge expansion of industrial iron founding from 1760 onwards.

[1][3] The conveyance of the output of the ironworks at Merthyr and Dowlais remained a serious issue,[citation needed] and eventually, in 1840, the Taff Vale Railway was built as the solution to the problem.

The connection to the NA&HR at Llancaiach was refused on account of objection by that company, so the Rhymney Railway was without the route to the docks that it sought.

In addition there was a 1+1⁄4-mile (2.0 km) branch from Crockherbtown Junction, immediately north of the present Cardiff Queen Street station, to the Bute Dock.

The approach to the junction with the TVR at Walnut Tree involved excavating a cutting to the extent of 1,000,000 cubic yards (760,000 m3) of earth.

[1] The Bute Trustees approached the Rhymney company with an offer: to lease the undertaking for 250 years retrospectively from 1 January 1860; to deal with all prior charges and in addition pay 5% on the ordinary stock from the beginning of 1865, rising gradually to that figure in the meantime.

The Bute Trustees evidently wished the Rhymney Railway to succeed, for they now agreed "in consideration of the capitalisation of the rent due to them" that dividends and other interest payments should have precedence over their own financial claims.

However the operating difficulties over the TVR section only became worse; and in 1863 the Rhymney Railway submitted a bill to make the direct line to Cardiff from Caerphilly.

The running powers between Walnut Tree and Cardiff would be extinguished, except for traffic to the new docks at Penarth, from the date of opening of the direct line.

[4] In fact the construction of the line proved very difficult at a time when money was becoming very scarce; as well as labour problems the Caerphilly Tunnel gave exceptional difficulty.

At 1 mile 173 yards (1.768 km) in length it was a hard drive, and an unknown spring in the middle during construction proved difficult to get under control.

The London and North Western Railway had certain rights over the NA&HR line, and seeing now an opportunity to get access to South Wales, it acquired a lease of the MT&AR in 1862.

[1][2][3][4] The LNWR now carried a lot of traffic to Cardiff and soon was receiving complaints that it had inadequate warehousing facilities there; it responded by building its own goods station at Tyndall Street.

At the same time the Great Western Railway had a corresponding idea, and on 29 March 1867 the Rhymney and the GWR agreed to build the line jointly.

The line had been extended to Middle Duffryn in the Aberdare Valley, and connected there with the Vale of Neath Railway, also now controlled by the GWR.

As part of the deal when the Taff Bargoed line was agreed with the GWR, the Rhymney Railway had secured running powers through Aberdare to Hirwaun.

[2][3] Apart from the new Cardiff to Caerphilly line, the Rhymney Railway was all single track, and the massive expansion of mineral traffic was becoming difficult to handle, leading to complaints about passenger train delays.

The company was forced to double the line between Caerphilly and Ystrad Mynach in 1872, and other expensive improvements, as well as enhancements to the locomotive fleet, had to follow.

The owners naturally wished to maximise the mineral traffic through their docks, and were held back by the difficult connection from the Taff Vale.

The new line crossed over the Taff Vale line and the Glamorganshire Canal shortly after leaving Quakers Yard GWR (later High Level) station, and at nearly 2 miles (3 km) south there was a branch on the east side to the Merthyr Vale group of coal pits, crossing the river on a long steel viaduct.

This line of 3+1⁄2 miles (5.6 km) was constructed to serve collieries that were developing in the valley; it had been authorised by the Rhymney Railway Act 1890 (53 & 54 Vict.

of 25 July 1890 this was to be converted into a proper branch line for goods and passengers, the sponsors being the Windsor Steam Coal Company and the Lewis Merthyr Consolidated Collieries.

From Penyrheol, 3⁄4 mile (1.2 km) from Aber, there was a continuous climb starting at 1 in 58 and steepening to 1 in 49 near the terminus, which comprised a station, extensive colliery sidings and a locomotive shed.

It had been opened with the object of providing competition for the mineral traffic of Cardiff Docks and the Taff Vale Railway, which together suffered from severe congestion, and in the view of their customers, high rates and poor service.

On 1 March 1911 the Cardiff Railway started to run a railmotor passenger service over their newly built line to Rhydyfelin.

For some years the mineral operation that had always been the primary business of the Rhymney Railway continued, but over later decades coal extraction in South Wales declined, steeply in the early 1980s.

At the same time local passenger operation increased in importance, notwithstanding substantial improvements to the road network in the valleys.

The part of the original main line between Taff's Well and Caerphilly, informally referred to as "the Big Hill", was closed on 21 June 1982, although the last train was a special on 23 October 1982.

Upon his retirement in 1904,[10] the Rhymney did a spring cleaning, which notably included the scrapping of his favourite engine, which he had been preserving for a number of years.

[12] The early locomotives were tender engines, whether for passenger or goods: Several original Rhymney Railway coaches have survived into the present day.