South Wales Main Line

The South Wales Main Line (Welsh: Prif Linell De Cymru), originally known as the London, Bristol and South Wales Direct Railway or simply as the Bristol and South Wales Direct Railway, is a branch of the Great Western Main Line in Great Britain. It diverges from the core London-Bristol line at Royal Wootton Bassett beyond Swindon, first calling at Bristol Parkway, after which the line continues through the Severn Tunnel into South Wales.

Much of the South Wales Main Line was built between the 1830s and 1886; originally trains to and from destinations in England ran via Chepstow, Gloucester and Stroud, joining the Great Western Main Line at Swindon. A more direct route was challenging yet desirable, leading to the construction of the line's most prominent civil engineering feature, the Severn Tunnel. Completed in 1886, it permitted a significant reduction in journey times between various destinations, especially after the construction of the Badminton Line in 1903. During the British Rail era, the line was incorporated into the Western Region and steam locomotives were replaced by diesel locomotives, such as the Intercity 125 high speed trainsets. During the 2010s, the line between Swindon and Cardiff Central was electrified using the 25 kV AC overhead system as part of the wider electrification of the Great Western Main Line; this has permitted the operation of electric traction between Cardiff and London for the first time to commence in 2020.

Presently, Great Western Railway operates Class 800 trains between London and South Wales, and "Castle class" High Speed Trains on services between Cardiff and South West England. CrossCountry provides services from Cardiff to Nottingham via Severn Tunnel Junction and thence the Gloucester to Newport Line via Gloucester and Birmingham. Transport for Wales operates services between South Wales and North Wales via the English Midlands on the line. Operations are mostly managed from the Wales Rail Operating Centre in Cardiff; digital signalling is to be rolled out.

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. At Gloucester, the South Wales Railway met with the Great Western Railway (GWR) which formed a route between London and South Wales after the opening of Brunel's Chepstow Railway Bridge in 1852. 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'.

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. During the early 1870s, GWR's chief engineer, Sir John Hawkshaw, developed his design for what would become the Severn Tunnel and the company obtained an Act of Parliament on 27 June 1872 that authorised the construction of the tunnel. The tunnel's construction was time consuming and disrupted by water infiltration, yet proceeded nonetheless.

The completed tunnel was opened to regular goods trains during September 1886; the first passenger train followed on 1 December 1886. The opening of a more direct route to and from South Wales led to trains from Swindon to Newport and beyond being thereafter routed via Bath, Bristol and the tunnel.

The route used today was established in 1903 with the building of what is often known as the Badminton Line. This involved the construction of about 33 miles (53 km) of new track, and tunnels at Alderton and Sodbury. The new line left the Bath line beyond Swindon at what is now Royal Wootton Bassett, rejoining the earlier route north of Bristol near Patchway. 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. This was the GWR's connection with trans-Atlantic ocean liner departures.

During the British Rail era, the line was incorporated into the Western Region and steam locomotives were replaced by diesel locomotives. Perhaps the most significant change occurred in 1976 in the form of the Intercity 125, a new high speed train fleet that regularly ran at speeds of up to 125 mph (201 km/h) that was first introduced in the Western Region. The Intercity 125 would remain in use into the twenty-first century and marked a considerable improvement in service.