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 of new track including two tunnels at Alderton and Sodbury between Wootton Bassett and Patchway. Not only did this provide a more direct route for traffic to and from South Wales, the gradient was easier for coal trains to negotiate, and it was perceived 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 liner departures for the United States of America.
There are four tracks from Severn Tunnel Junction through Newport to Cardiff Central, with two tracks on the remaining sections. Multiple-aspect signals are controlled from several power signal boxes including Swindon, Bristol and two in Cardiff.
The maximum line speed from London to Coalpit Heath is 125 mph (200 km/h); 90 mph (145 km/h) from Coalpit Heath to Newport; 90 mph (145 km/h) from Newport to east of Bridgend; 75 mph (120 km/h) from east of Bridgend to Swansea Loop North junction (with a small section of 100 mph (160 km/h) track through Pyle station); and 40 mph (65 km/h) from Swansea Loop North Junction to Swansea.
Traffic levels on the Great Western Main Line are rising faster than national average, with continued increases predicted. The now defunct Strategic Rail Authority produced a Route Utilisation Strategy for the Great Western Main Line in 2005 to propose ways of meeting this demand, Network Rail plan to implement a new study in 2008. In the meantime, their 2007 Business Plan includes the provision of extra platform capacity at Cardiff Central, Newport and Bristol Parkway, also resignalling and line speed improvements in South Wales, most of which would be delivered in 2010–2014.
The South Wales Main Line is one of the last of the major inter-city routes in Great Britain to remain un-electrified. The government announced in July 2009 a scheme to electrify the South Wales Main Line as part of a wider scheme of electrification on the Great Western Main Line. By 2017, the line from London to Cardiff will be electrified.
The new Hitachi Super Express trains planned for the Great Western inter-city services will now be predominantly electric units instead of the planned diesel units. However, a proportion of the fleet will be using dual power source electro-diesel bi-mode trains, which will enable services to operate before line electrification is complete. The bi-mode trains will allow inter-city services to operate from London all the way to Carmarthen in the future. The new Super Express trains will bring about an estimated 15% increased capacity during the morning peak hours. Electrification will cut journey times between Swansea and London by an estimated 20 minutes, although electrification will not extend West of Swansea to Carmarthen or Pembroke Dock, and services on the line to Brighton, Portsmouth Harbour and Taunton will continue to be operated by Diesel Trains, as the Bristol to Exeter Line and the Wessex Main Line will not be electrified.