The South West Main Line (SWML) is a 143-mile (230 km) major railway line between Waterloo station in central London and Weymouth on the south coast of England. A predominantly passenger line, it serves many commuter areas including south western suburbs of London and the conurbations based on Southampton and Bournemouth. It runs through the counties of Surrey, Hampshire and Dorset. It forms the core of the network built by the London and South Western Railway, today mostly operated by South Western Railway.
Operating speeds on much of the line are relatively high, with long stretches cleared for up to 100 mph (160 km/h) running. The line has four tracks for most of its length between Waterloo and Worting Junction, south west of Basingstoke; most of the rest is double track. A couple of miles from the Waterloo terminus, the line runs briefly alongside the Brighton Main Line west branch out of London Victoria, including through Clapham Junction.