A predominantly passenger line, it serves many commuter areas including south western suburbs of London and the conurbations based on Southampton and Bournemouth.

[2][3] The following February, Francis Giles was commissioned to survey the route and a formal scheme, which also included the construction of new docks on the Solent, was presented at a public meeting on 6 April 1831.

[4] Giles, who was familiar with the west Surrey and north Hampshire area, having worked as an engineer for the Basingstoke Canal, examined potential routes via Guildford and Farnham.

[8] A formal ceremony to mark the start of construction took place at Shapley Heath, near Winchfield, on 6 October 1834, and the contracts for the earthworks and bridges had been let by the end of that month.

[17][b] On 21 May 1838, the first section of the South West Main Line, between Nine Elms and Woking Common, opened with intermediate stations at Wandsworth (later replaced by Clapham Junction), Wimbledon, Kingston (now Surbiton), Ditton Marsh (now Esher), Walton and Weybridge.

[25] The opening ceremony for the completed line from Nine Elms to Southampton took place on 11 May 1840, with a directors' train leaving London at 8 am and arriving at the permanent terminus, designed by William Tite, around three hours later.

[28] Three years later, the company acquired the Richmond and West End Railway, which had been authorised to build its own tracks alongside the London–Southampton line between Clapham Junction and Waterloo.

[30] The four-track line was carried on a viaduct of 290 arches,[29] which followed a sinuous path to avoid encroaching on Vauxhall Gardens, the local gas works, and Lambeth Palace.

[32][33] In February 1844, a group of Dorset businessmen, led by Charles Castleman, a solicitor from Wimbourne, proposed a railway line linking Southampton to Dorchester.

[25] Surveyed by William Moorsom, the Southampton and Dorchester Railway (S&DR) was to take an indirect route to serve as many towns as possible, and its sinuous nature gave rise to the nickname "Castleman's Corkscrew".

[36] Castleman hoped to persuade the LSWR to operate the route, but the company refused, fearing that it would not be able to build any further lines west of Salisbury if it accepted.

[40] The Southampton and Dorchester Railway Act 1845 was granted royal assent on 21 July 1845 and, the following month, Samuel Morton Peto was contracted to build the line.

[41] The link to the LSWR opened on 29 July that year, allowing through running of trains between London and Dorset, albeit with a reversal at the Southampton terminus station.

[56] The South Western (Bournemouth & c.) Act 1883, passed on 20 August 1883, authorised the construction of a cut-off line between Brockenhurst and Christchurch, which would reduce the distance between the two by around 7+3⁄4 mi (12.5 km) compared to the existing route via Ringwood.

[57] Progress on the 10 mi 38 ch (16.9 km) cut-off line was hindered by poor weather in spring 1887, which flooded cuttings and damaged embankments.

[81] The GWR completed its branch from Reading to Basingstoke in November 1848, but there was no direct connection with the South West Main Line until mixed-gauge tracks were brought into use on 22 December 1856.

[89][91] Following the opening of Waterloo station and Nine Elms Viaduct, the next part of the South West Main Line to be four-tracked was the section from Wimbledon to New Malden Junction.

[92] The new tracks ran from Kingston via a new station at Norbiton, passing beneath the South West Main Line before turning to run parallel with it for around 2+3⁄4 mi (4.4 km).

[95] The stretch from Surbiton to Hampton Court Junction was widened to four tracks on 29 July 1883, in part to accommodate traffic using the New Guildford line, which opened in February 1885.

[103] As part of the widening works, Pirbright Junction, where the Alton line diverges, was grade separated and the new flyover for Aldershot to London trains opened on 30 June 1901.

[104][105] The project also required the lengthening of Frimley Aqueduct, blocking the Basingstoke Canal for a five-month period,[101][102] and the relocation of Fleet station around 250 yd (230 m) to the west.

In 1914, to resolve these operating constraints, the LSWR proposed constructing a flyover in the Vauxhall area, but the outbreak of the First World War meant that the project did not progress.

[115] By the end of 1847, an electrical telegraph system had been installed along the entire length of the line, giving railway staff the ability to communicate the positions of trains.

[117] In March 1895, the LSWR board agreed to install William Robert Sykes's system of interlocking mechanical block signals between Earlsfield and Hampton Court Junction at a cost of £1,975 (equivalent to £288000 in 2023).

[130][131] Over the next five years, the company investigated electrifying some of its surface routes and, in 1903, the resident engineer, J. Jacomb-Hood, told the board that running an intensive electric service over the slow lines between Waterloo and Hampton Court Junction would be no more expensive than the existing steam operation.

[132] Over the course of the next six years, passenger numbers on the LSWR declined by 1+1⁄4 million and, in 1912, the company estimated that its annual income would have been around £100,000 higher, had its suburban lines been electrified.

[133] That year, the company authorised the laying of the third rail along 47 route miles (76 km),[134] and the first electric trains operated in public service to Shepperton on 30 January 1916, running along the South West Main Line between Waterloo and New Malden Junction.

It was planned originally that a 3-car single-ended push-pull trailer set formed DTC+BFK+RU would work with a Class 74 to cover 4-Rep heavy maintenance, but this formation never operated and was replaced by the 8-Vab unit 8001; the three cars remained as spares.

[147] To reduce costs, a 5 mi (8.0 km) stretch of line was singled between Moreton and Dorchester South,[148] and the capacity of the 11 kV supply from the National Grid limited train lengths to a maximum of five coaches.

In July 2011, Network Rail in its London & South East Route Utilisation Strategy (RUS) recommended adding a fifth track to the four-track stretch of line between Clapham Junction and Surbiton.