The Swansea District Line is a section of railway line running through the northern part of Swansea, and is primarily used for freight transportation, although minimal passenger services also traverse the route. It was built by the Great Western Railway in 1912 to provide a faster and less steeply graded route between London and Fishguard, in connection with the recently opened harbour at the latter place. It can thereby claim to be the very last mainline railway to have been built in Wales. The double track line runs from Cwrt Sart junction at Briton Ferry on the South Wales Main Line to Morlais junction near Pontarddulais on the Heart of Wales Line.
The line is currently used mainly for freight traffic, but Arriva Trains Wales run two or three services along the line in each direction on Mondays to Saturdays, one of which is the daytime boat train between Fishguard Harbour and Cardiff, which extends to Cheltenham Spa in the eastbound direction. The boat train service is run in connection with the Stena Line ferry to/from Rosslare in Ireland. By taking the Swansea District Line, these passenger services bypass Neath, Swansea and Gowerton railway stations, thereby also avoiding the single track section through Gowerton station, which was redoubled in 2013.
Network Rail are considering rationalisation of this line following replacement of the Loughor railway viaduct and re-doubling of the track at Gowerton, work which was completed in 2013. This could mean more traffic being diverted via the South Wales Main Line and Gowerton station.