A prospectus was issued in 1844 to build a railway through south Wales from a junction with the Great Western Railway at Standish. The proposed route would cross the River Severn west of Gloucester, and run through south Wales to Fishguard, where it would connect with ferries to southern Ireland. This undertaking became known as the South Wales Railway and the Great Western Railway subscribed £600,000 of the £2,800,000 capital.
Objections to the long bridge over the River Severn resulted in the South Wales Railway ending the eastern end of its line at Grange Court. At Grange Court an end-on junction with the Gloucester & Forest of Dean Railway was made, which connected the South Wales line to the rest of the broad gauge network via Gloucester. The revised route meant an additional 18 miles (29 kms) to the journey between south Wales and London.
The line was opened between Chepstow and Swansea on 18 June 1850 and trains were operated by the Great Western Railway under a lease agreement which was destined to cause ongoing disputes between the two companies. At the eastern end of the line the connection to Gloucester and London was completed in July 1852 when the bridge at Chepstow was finally finished. Construction of the line west of Swansea was delayed, partly due to the financial problems of the late 1840s, and partly due to the abandoning of construction of the Irish railways that would connect with the Fishguard ferries at Waterford. The western terminus of the line was changed from Fishguard to New Milford (Neyland) and the line west of Swansea was built in stages, reaching New Milford in April 1856.
The South Wales Railway connected with several other Broad Gauge lines which brought coal and mineral traffic to the line. The rapid expansion of coal mining in the region meant the South Wales Railway was hard pressed to cope due to a shortage of wagons. The ongoing dispute over traffic arrangements with the Great Western Railway was finally resolved in January 1862, when the two companies merged.
This railway was incorporated in 1846 to build a line from a junction with the South Wales Railway at Neath to Merthyr Tydvil, with a branch to Aberdare. The line required significant engineering works and was opened between Neath and Aberdare on 24 September 1851. The "main line" to Merthyr was opened 2 November 1853 after being delayed by the construction of the 2,495 yd (2.3km) tunnel at Merthyr. The Vale of Neath Railway operated its own trains, eventually owing 19 locomotives.
The Aberdare region was rich in high quality coal and the opening of the railway saw a rapid increase in the number of collieries in the area. Most coal was taken to Neath where the South Wales Railway took it the ports at Swansea or Britton Ferry. The shortage of locomotives on the South Wales Railway resulted in the Vale of Neath Railway being given running rights over the South Wales Railway between Neath and Swansea in March 1861. This was followed in July 1863 by the opening of the Vale of Neath Railway's own line between Neath and Swansea docks.
In 1864 the line was converted to mixed gauge to accommodate traffic from the standard gauge West Midland Railway (part of the Great Western Railway since 1860) which formed an end-on junction with the Vale of Neath Railway near Aberdare. On 31 January 1865 the Vale of Neath Railway merged with the Great Western Railway.
A number of independent feeder railways were built with broad gauge track, all operated by the Great Western Railway.
Llynvi & Ogmore Railway - a 10 mile (16km) railway between Bridgend and Porthcawl harbour opened in 1861. It was converted to mixed gauge in stages between 1866 and 1868 and standard gauge in May 1872. It became part of the Great Western Railway in June 1873.
Carmarthen & Cardigan Railway - a 18½ mile (30km) railway from a junction with the South Wales Railway at Carmarthen to Llandyssil. The line was opened in stages between 1860 and 1864. The line was converted to mixed gauge in 1866 and standard gauge in 1872. The railway became part of the Great Western Railway in August 1881 and eventually reached Cardigan in 1886.
South Wales Mineral Railway - a 12½ mile (20km) railway between Briton Ferry Junction (with the South Wales Railway) to Glyncorrwg completed in 1863. It was converted to standard gauge in May 1872 and became part of the Great Western Railway in 1907.