The South Wales Railway (Welsh: Rheilffordd De Cymru) was a main line railway which opened in stages from 1850, connecting the Great Western Railway from Gloucester to South Wales. It was constructed on the broad gauge. An original aspiration was to reach Fishguard to engender an Irish ferry transit and transatlantic trade, but the latter did not materialise for many years, and never became an important sector of the business. Neyland was the western terminus of the line until 1906.
The company amalgamated with the Great Western Railway in 1863 and the track was converted to narrow (standard) gauge in 1873. In 1922–1923, most of the independent Welsh railways were constituents of the new enlarged Great Western Railway, enabling rationalisation and benefits of scale. Nearly all of the original main line of the South Wales Railway remains in use at present (2020).