The conveyance of coal for export and for transport away from South Wales began to dominate and the docks in Cardiff and the approach railway became extremely congested.

The decline in the coal and iron industries took its toll on the mainstay of the network, but passenger trains still operate on most of the main line sections.

In 1767, Anthony Bacon of Cyfarthfa persuaded his fellow ironmasters to join in a scheme to build a trunk roadway from Merthyr Tydfil to Cardiff.

[7] For the first decades of the nineteenth century, the transport situation for Merthyr and the surrounding area changed relatively little; the Glamorganshire Canal prospered, but became overwhelmed by the volume of trade.

The Glamorganshire Canal Company opposed the bill, but it was passed and obtained royal assent on 21 June 1836 as the Taff Vale Railway Act 1836 (6 & 7 Will.

The general gradients, the inclined planes, and still more the nature and the immediate extent of the peculiar class of traffic to which the line must always be devoted, not only render high speeds unnecessary, but must almost prevent their being attempted, while the same causes operate to diminish any advantage that may be gained in reducing friction by increased diameter of carriage wheels.

[b] The curves also which the nature of the ground render unavoidable would be unfit for a wider gauge…[15]The construction of the line posed no great engineering challenges as its course followed the valley of the River Taff.

At Quakers Yard there was a sudden steep change of ground level and Brunel used 50-horsepower (37 kW) stationary winding engines; the inclined section was 1⁄2 mile (800 m) in length with gradients of 1 in 19 and 1 in 22.

[12] As well as the avoiding the use of broad gauge, Brunel adopted a different form of track for the line: "parallel rails" weighing 55 pounds per yard (27 kg/m), fixed in chairs with compressed wooden keys.

[c] Passenger stations were at Cardiff, Llandaff, Pentyrch (later Radyr), Taffs Well, Newbridge (later Pontypridd), Navigation House, Troed-y-rhiw and Merthyr.

In the years immediately following the railway's authorisation, the rich seams of high-quality coal in the Aberdare area came into prominence and began to outshine the Merthyr trade.

[12][3] In 1847 the Newport, Abergavenny and Hereford Railway obtained powers to build its Taff Vale Extension westwards from Pontypool to connect the TVR at Quakers Yard.

The branch line from Pontypridd to the tramroad leading to the collieries called Dinas had been opened in 1841 which stimulated interest in mining in the valley.

At the same time a 77-chain (1.5 km) extension from Porth to Ynyshir was opened for mineral traffic; that stub was extended to Ferndale in 1856 and later to Maerdy, which at 900 feet (270 m) above sea level was the most elevated location on the TVR system.

[12] In 1854 the Eirw Branch was opened; under 1 mile (1.6 km) long it left the Rhondda line at Trehafod to serve nearby collieries.

[12] The Cogan Pill branch authorised in the original act was deferred, chiefly due to considerable opposition by the Marquis of Bute.

c. cxxii) enabled construction of a tidal harbour at Penarth, southwest of Cardiff, and an approach railway from Radyr on the TVR line.

The railway leading to Tidal Harbour, east of Cardiff, was just over 6 miles (10 km) long and opened in August 1859, but the dock was not completed until 1865.

[clarification needed] The opening of the Penarth Docks proved a huge benefit to the company, not only in bringing in direct revenue, but in easing congestion on the main line as mineral trains were diverted from Radyr.

The mineral output from the mines there could now more conveniently be brought out via Quakers Yard, avoiding the incline on the Llancaiach branch, which was closed except for a short section at the north end.

In 1872 the company presented a parliamentary bill to build on that,[clarification needed] making a new railway to Llancaiach, with new connections to the Taff Vale Extension line.

[17] The TVR's original station in Merthyr Tydfil at Plymouth Street was opened on 12 April 1841 and was a short distance south of the town.

The Company obtained authority to build a northwest to southeast diagonal line across the Llantrisant and Taff Vale Junction Railway system to handle this traffic.

Financial difficulties delayed construction and in the meantime the South Wales main line of the GWR was converted to standard gauge.

[17] In 1900 a south curve connection to the main line was opened but the TVR was concerned about congestion at Pontypridd and held off from starting the southward passenger service.

[17] The original Llancaiach branch, opened in 1841, had left the Merthyr main line at Stormstown Junction, just south of Abercynon, crossed the River Taff and swung east to reach its objective.

Later that year, the opening of the Vale of Glamorgan Railway connecting Aberthaw directly to Penarth and Cardiff took much of the potential mineral traffic away from this line.

The intention was to adopt a low-cost means of serving wayside communities by opening very basic stopping places (referred to as "platforms" rather than "stations") and having a more frequent timetable.

The end of World War II had brought to notice what had become inefficient working methods and British Railways immediately set about some closures.

Other main depots were at Abercynon, Aberdare, Coke Ovens (near Pontypridd), Cowbridge, Ferndale, Merthyr, Penarth Dock, Radyr and Treherbert (which had a sub-shed at Pwllyrhebog).