The Central Wales Line was a creation of the LNWR in its efforts to reach Swansea. The process proved complicated and involved a number of railway companies and some financial disasters. At the southern end it was the Llanelly Railway, one of the earliest in Britain founded in 1829, which opened a line as far as Pontardulais in 1839. By 1857 the railway had reached Llandilo. A separate company, the Vale of Towy Railway, opened between Llandilo and Llandovery in the same year, and was leased to the Llanelly.

Meanwhile at the other end of the line, in 1858 the Knighton Railway was formed to build a line from Craven Arms, on the north/south Shrewsbury to Hereford line, as far as Knighton. A year later the Central Wales Railway was formed to take the railway on to Llandrindod, and in 1860 a further company, the Central Wales Extension Railway was formed to make the link with the Llanelly at Llandovery. Behind the scenes was the LNWR.

It took until 1868 to complete the line with a branch from Pontardulais to Swansea. In that same year the LNWR took over the Knighton, Central Wales, and Central Wales Extension Railways and took a half share in the Vale of Towy. Part of the reason for the time taken was the construction of the Sugar Loaf tunnel and the Cynghordy Viaduct just to the north of Llandovery on the Extension line. From that date the LNWR had access to Swansea.

Knucklas is a small village in former Radnorshire which was one of the rotten boroughs entitled until the 1832 Reform Act to return two MPs to Westminster.

The viaduct at Knucklas has 13 arches and a maximum height of 23 m. (75 ft.) It is 174 m. long across a tributary of the river Teme. It is one of the most picturesque viaducts in Britain as it has a crenellated parapet with round towers at each end and square towers at the buttresses. The whole is in a rough stone and the effect is as though it had been there for centuries. This may have been the intention of the local landowner who is believed to have commissioned the decorative work.