Taff Vale Railway - Main Line: Cardiff - Merthyr Tydfil

Merthyr Tydfil

Construction was started in 1836, and the stretch from Cardiff to Navigation House (later named Abercynon) was opened in a formal ceremony on 9 October 1840, with public services starting the next day. The stretch from Abercynon to Merthyr was opened on 12 April 1841. The railway was Single track for its entire length, with passing only possible at the six intermediate stations: Llandaff, Pentyrch (renamed Radyr), Taff's Well, Newbridge (renamed Pontypridd), Navigation House (now Abercynon), and Troedyrhiw. Brunel, the chief engineer, had chosen a narrower gauge (4 ft 8 1⁄₂ in/1,435 mm) rather than the 7 ft 0 1⁄₄ in (2,140 mm) gauge he would use for his Great Western Railway in order to fit the railway into the narrow, curvy space allowed to him by the River Taff valley.

The engineering of the 24 miles (39 km) main line was relatively straightforward. The line mostly followed the course of the Taff valley, with largely gentle gradients, except for a short portion between Navigation House and Quaker's Yard. This half-mile stretch, at a gradient of between 1 in 19 and 1 in 22, was worked by winding engines: trains were timed to meet at the incline, and locomotives would exchange their trains of carriages instead of traversing the incline. Two stone viaducts were built by Brunel on the route. The first, at Pontypridd, crosses the River Rhondda, and the second bridges the Taff valley between Goetre Coed and Quaker's Yard.

A station was opened at Maesmawr near Tonteg in April 1845, but was unsuccessful and closed three months later. Its contemporary station in nearby Trefforest remains in use to this day. Incline Top station (later Top of Incline) opened in 1846. In the same year, Navigation House (Abercynon) station was rebuilt to accommodate the new branch line to Aberdare, and renamed Aberdare Junction.

An act passed in 1857 gave the railway authority for a number of improvements over the coming years. The line was doubled throughout from 1858 to 1862, and later quadrupled between Pontypridd and Cardiff to accommodate the growth in traffic. New viaducts were built alongside the existing structures at Pontypridd and Quaker's Yard to carry the second line. In 1864 work started on bypassing the incline with a gentler bank (1 in 40). This required significant earthworks, and inclement weather meant that it was not completed until August 1867. Top of Incline station had already closed in 1858, replaced with a new station at Quaker's Yard. Pontypridd station saw platforms added and extended during the 1860s, as it became the busiest station on the Taff Vale network. Freight relief lines were built there in the 1890s, and from 1907 to 1914 the station was rebuilt as a single island platform with numerous bays. This work included raising the level of the entire station by nearly 5 feet (1.5 m).

The TVR's original station in Merthyr at Plymouth Street was opened on 12 April 1841, and was a short distance south of the town. This was joined in 1853 by the Vale of Neath's High Street station. A short joint line (TVR and GWR) was built to connect the TVR line to the new station in 1877. A year later, in August 1878, the Taff Vale transferred all of its passenger services to the High Street station, and used Plymouth Street as a goods depot instead. High Street station was thus the only passenger station in Merthyr, and was used by a total of six separate companies prior to the 1923 grouping. The TVR also opened stations at Merthyr Vale in 1883 and Pentrebach in 1886. The GWR later opened a station at Treforest Industrial Estate in 1942.

Extensive underground coal mining in the Quaker's Yard area led to troubles with subsidence, particularly on the Quaker's Yard viaduct. This was first noted in 1900, and the masonry was reinforced with steel girders in 1902. but subsidence continued. The successor GWR reinforced the viaduct throughout with timber from 1924 onwards, and a 1931 Chief Engineer's report noted surface subsidence of at least 1 foot 6 inches (0.46 m), and compensation was sought from the nearby collieries. Further work had to be done by the end of the decade. Despite the end of mining in the area, the bridge is actively monitored for subsidence to this day.

Closures in the mid 20th century left the Taff Vale as the only railway route to Merthyr Tydfil. Plymouth Street goods station was closed in 1968. The line from Black Lion signal box (Merthyr Vale) to Merthyr Tydfil was singled in February 1971. The quadrupled line south of Pontypridd became surplus to requirement, and two of the lines were lifted in 1980, leaving a double line in service.