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.

The first written proposal for a railway line linking London and Southampton was published on 23 October 1830 by a group chaired by the Southampton MP Abel Rous Dottin. 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. 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. However, his favoured alignment, via Basingstoke, was chosen to facilitate a future western branch to Bath and Bristol. His detailed survey was published on 5 December 1833 and the construction of the line from London to Southampton was approved in the London and South Western Railway Act 1834 on 25 July the following year.

Under the terms of the 1834 act, the London and Southampton Railway company was authorised to issue shares to the value of £1 million (equivalent to £120 million in 2023) and to borrow a further £330,000. Around half of the money was raised from businessmen from Lancashire, with much of the remainder provided by landowners from south Hampshire. Giles had estimated that his route would take three years to build at a cost of £800,000–£900,000. 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. Giles's strategy was to use numerous small-scale, local contractors, with work taking place simultaneously on multiple sites along the route. By February 1836, just under 10 mi (16 km) had been finished, with a further 12 mi (19 km) completed by the end of August 1836. Shareholder dissatisfaction with progress led to Giles's resignation on 13 January 1837 and his replacement by Joseph Locke.

Locke assessed the progress of the construction works and estimated that around £1.7 million (£200 million in 2023) would be required to complete the line. He dismissed many of the smaller contractors, awarding much of the remaining work to Thomas Brassey. He also instituted a more rigorous supervision system by his assistant engineers. On 30 June 1837, the London and South Western Railway Deviations Act 1837 was passed, enabling the company to raise further capital and authorising deviations to the route. 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. The extension westwards to Shapley Heath (now Winchfield), with a station at Farnborough, opened on 24 September 1838.

On 14 June 1839, the London and Southampton Railway adopted the new name of the London and South Western Railway (LSWR). Four days earlier, the first trains had run from Winchester to a temporary terminus north of Southampton at Northern Road (now Northam). The extension from Shapley Heath to Basingstoke also opened on 10 June 1839, and the section between Basingstoke and Winchester, which required three tunnels, was finished in May the following year. 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.

The LSWR did not intend Nine Elms to be its permanent northern terminus, and in 1844 an extension of 1+3⁄4 mi (2.8 km) to a new station at Waterloo Bridge was authorised. 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. Construction of the line north of Nine Elms, known initially as the "Metropolitan extension", began on 11 July 1848. The four-track line was carried on a viaduct of 290 arches, which followed a sinuous path to avoid encroaching on Vauxhall Gardens, the local gas works, and Lambeth Palace. The new terminus, designed by Tite and with four platforms, was initially known by a variety of names including York Road, Waterloo Bridge and Waterloo. On the day that Waterloo was opened, the Nine Elms terminus closed to passengers and was replaced by Vauxhall station.

In February 1844, a group of Dorset businessmen, led by Charles Castleman, a solicitor from Wimbourne, proposed a railway line linking Southampton to Dorchester. 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". In particular the line would run via Ringwood, bypassing Bournemouth, which was a small village at the time. Poole, with a population of only 6,000, would be served by a branch from a station at Hamworthy. 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. Instead, the Great Western Railway (GWR) agreed to run services using broad-gauge trains. Concerned at this development, the LSWR proposed a rival scheme, the Salisbury & Dorsetshire Railway, which would have paralleled much of Castleman's line. Both proposals were considered by the Board of Trade, which favoured the S&DR, but which also indicated that the route should be standard gauge. The LSWR agreed to lease Castleman's line for an annual sum of £20,000 (equivalent to £2.5 million in 2023).