The London and South Western Railway originated as a renaming of the London and Southampton Railway, which opened in May 1840 to connect the port of Southampton with London. Its original London terminus was Nine Elms, on the south bank of the river Thames, the route being laid through Wimbledon, Surbiton, Woking, Basingstoke and Winchester, using what became the standard track gauge of 4 ft 8 1⁄2 in (1,435 mm).
The railway was an immediate success, and this encouraged the company to think of extensions, to Windsor, to Gosport (for Portsmouth) and to Salisbury. The company then saw potential from the area westward, which put it in direct competition with the Great Western Railway: it was important to secure lines and stations to seek to keep the competitor out. As the Great Western Railway used the broad gauge (7 ft or 2,134 mm), any gauge adopted by independent smaller lines dictated their permissibility for joint running, and this territorial competition became known as the gauge wars. The Nine Elms terminus was obviously inconvenient to most Londoners and the line was extended north-eastwards to Waterloo via the Nine Elms to Waterloo Viaduct in 1848; later the LSWR built its own tube railway—the Waterloo & City line—to connect to City station close to the Bank of England building in the City of London.[note 1]
The Great Western Railway secured access early on to Exeter and Plymouth through its allied companies, and the LSWR aspired to build its own competing route to reach Devon and Cornwall, which would offer considerable traffic potential. It made a slow start but eventually had its own line from Basingstoke to Salisbury and Exeter, continuing by a northerly arc to Plymouth, and to north Devon and north Cornwall. Coming later than the Great Western to the area, it never achieved the solid prosperity there of its broad gauge neighbour.
The Southampton line had been extended to Weymouth via Ringwood, and the LSWR consolidated its home area building branches closer to London, and direct lines to Portsmouth, and to Reading. It also became joint owner, with the Midland Railway, of the Somerset and Dorset Railway, responsible for infrastructure and coaching stock on the latterly famous route. Shipping became significant also, with passenger and freight services to the Channel Islands, to Saint-Malo in France, and to the Isle of Wight.
In the twentieth century, it embarked on a programme of electrifying the suburban routes, at 600 V DC using a third rail. Eventually this covered the entire suburban area. Freight traffic, especially from the West Country was important, but the emphasis on suburban electrification led to weaker development of steam traction for fast passenger and goods services to Devon and Cornwall, and to Portsmouth, Bournemouth and Weymouth.
At the grouping of the railways, the LSWR amalgamated with other railways to create the Southern Railway, and the independent Isle of Wight railways were absorbed, becoming part of the former LSWR section within the Southern Railway. Its enlightened and unorthodox Chief Mechanical Engineer, Oliver Bulleid, put in hand the construction of a fleet of powerful express steam locomotives, the Merchant Navy class, followed by a larger fleet of so-called light pacifics, built with lighter axle loading to give access to branch lines with weaker track and bridge strengths; this enabled radical improvements to main line passenger services, and the streamlined profile of the new fleet made an impact as a modern design, and it remained an iconic image. At the same time they revolutionised express passenger train speeds to Weymouth and the West Country, although their technical innovation incorporated a number of difficulties. Electrification of the Portsmouth line was now carried out.
Capital infrastructure works were also undertaken, including the Feltham marshalling yard, major improvements to Southampton Docks and Waterloo station, a new locomotive workshop at Eastleigh, and grade separated junctions on the main line, as well as signalling modernisation schemes. A concrete manufacturing works was established at Exmouth Junction (Exeter) producing standardised pre-cast components such as platform units, lamp posts and platelayers' huts; the designs became familiar throughout Southern Railway territory.
Nationalisation of the railways in 1948 brought relatively little immediate change to the former LSWR system, now part of the Southern Operating Area of British Railways, later the Southern Region, although national centralisation of locomotive design made Bulleid's position untenable and he retired. However, in 1966 the geographical limits of the British Railways regions was rationalised, and the Devon and Cornwall lines were transferred to the Western Region. Many of the branch lines had declined in traffic in the 1950s and were considered uneconomic, and following the Beeching Report, the Reshaping of British Railways, many of the branches were closed, as was the Plymouth main line.
The Bournemouth line was electrified in 1967, at first with converted steam coaching stock, later replaced by purpose built stock; the electrification was extended to Weymouth. In recent years the system has remained constant, with gradually increasing train frequency from about 1990 now forming a limitation.
When international train services from London to Paris and Brussels were initiated in 1994, they required space for very long trains, and this was provided at Waterloo: the Eurostar terminal was built on the north side of the station with the international trains using the first three miles (4.8 kilometres) of the LSWR main line before diverging. The services were transferred to St Pancras International in 2007 and at present (2017) the Eurostar terminal, having sat dormaint for a decade, is being developed for re-opening and use by Windsor & Reading services.