South Western Main Line

The South Western Main Line (SWML) is a major British railway route between London Waterloo and Weymouth on the south coast of England. It serves many important commuter areas, as well as the conurbations based on Southampton and Bournemouth. It runs through Greater London, Surrey, Hampshire and Dorset.

It has many branches, including the lines to Windsor and Reading (the "Windsor lines"), Dorking, Guildford, Portsmouth, Kingston upon Thames and the West of England Main Line, which shares the route between London and Basingstoke. Together with these, it forms part of the network built by the London and South Western Railway, today mostly operated by South West Trains. Network Rail refers to it as the South West Mainline.

Much of the line is relatively high-speed, with large stretches cleared for 100 mph (160 km/h) running. The London end of the line has as many as eight tracks, but this narrows to four through most of the suburbs and continues this way until Worting Junction near Basingstoke, from which point most of the line is two tracks. A couple of miles from the Waterloo terminus, the line runs briefly alongside the Brighton Main Line out of London Victoria, and both routes pass through Clapham Junction - the busiest station in Europe by railway traffic.