A more direct route was challenging yet desirable, leading to the construction of the line's most prominent civil engineering features, the Severn Tunnel.

The South Wales Railway was built to carry traffic from Gloucester to the strategic port of Milford Haven and capture the lucrative transatlantic maritime trade as well as communication with Ireland.

[3] The original route of the GWR left the Bristol-bound Great Western Main Line at Swindon, proceeding via Stroud, Gloucester and Chepstow before rejoining the present line at Severn Tunnel Junction; this circular route gave rise to the nickname 'Great Way Round'.

[4][5] GWR officials realised that the journey time between the South Wales Main Line and the Great Western Main Line could be significantly shortened by the construction of a tunnel directly underneath the River Severn, which would be faster than the ferry service between Portskewett, Monmouthshire and New Passage, Gloucestershire.

Not only did this provide a more direct route for traffic to and from South Wales, the gradients were easier for coal trains to negotiate, and it was thought that the line would be a boost to what was, at the time of building, the expanding port of Fishguard.

[13][14] In 2005, the Strategic Rail Authority produced a Route Utilisation Strategy for the Great Western Main Line in 2005 to propose ways of meeting increased traffic levels.

Network Rail's 2007 Business Plan included the provision of extra platform capacity at Cardiff Central, Newport and Bristol Parkway, together with resignalling and line speed improvements in South Wales, most of which would be delivered in 2010–2014.

The South Wales Main Line was one of the last of the major inter-city routes in Great Britain to remain un-electrified.

The line from London to Cardiff was fully electrified by Christmas 2019,[15][16] the first electric trains began operating through the Severn Tunnel in June 2020.