During the First World War the government took control of the railways to co-ordinate the war effort. After the war it was decided that the railway companies could not competitively return to their prior state, and so the 1921 Railways Act merged the 120 existing railway companies into four companies, which became known as the ‘Big Four’’. Founded in 1923, the Southern Railway took over the railways of South-East England and came to consist of five major railway companies and 14 small ones. The Southern Railway had works at Ashford, Brighton, Lancing and Eastleigh. The first chairman of the Southern Railway was Brigadier-General Sir Hugh Drummond. Initially the general managers of the three main constituent companies, namely the London & South Western Railway (LSWR), London, Brighton & South Coast Railway (LBSCR) and South Eastern and Chatham Railway (SECR) worked together to lead the Southern Railway during its first year of operation. From 1 January 1924 it was decided that railway would be led by one general manager and one chief mechanical engineer. The first holders of these posts were Sir Herbert Ashcombe Walker, formerly manager of the LSWR and Richard Maunsell previously the Chief Mechanical Engineer of the SECR. Walker was succeeded by Gilbert Szlumper, then Sir Eustace Missenden and the last general manager prior to nationalisation was Sir John Elliott. The Southern Railway's operating structure was based on geographical divisions, initially these were London (East), London (West), Eastern (Dover) Southern (Brighton), Central (Southampton), and Western (Exeter). The Southern Railway was the smallest of the Big Four, and it relied primarily on passenger traffic. The Southern Railway’s predecessors had begun the process of electrification around suburban London where there was a high volume of passenger traffic. This programme was continued by the Southern Railway and was approved by its first AGM in March 1924. A rolling programme of electrification continued throughout Southern Railway’s operation, and on 30 December 1932 the newly electrified line to Brighton was opened. Despite Southern Railway’s increasing use of electric trains, steam locomotives continued to be used, especially for goods trains, until the nationalisation of the railways in 1948. As well as operating rail traffic, the Southern Railway owned and operated docks and harbours along the South Coast, the most important of which was Southampton Docks. It also ran passenger steamers across the English Channel to France. The Southern Railway operated air services to the Channel Islands from a number of airfields in the South-East and had stations at these airfields to allow passengers to transfer. The 1947 Transport Act nationalised the railways, and the Southern Railway came to be run by the Railway Executive as part of the new British Transport Commission. The Southern Railway’s rail operations were taken over by the Southern Region of British Railways.

The London and South Western Railway Company (LSWR) opened their first line in stages in from London to Southampton in 1838 under the name London and Southampton Railway Company. A year later the company name was changed to the LSWR. By the early 1900s, the line had been extended and ran from London to Plymouth via Yeovil, Exeter and Southampton. Some of the branch lines of the LSWR, further into Southern counties, were in direct competition with the Great Western Railway Company and others were worked in partnership with other railway companies such as the Somerset and Dorset Joint Railway. Nine Elms in London was the main depot of the LSWR, until Waterloo Station was opened by the company in 1848. Nine Elms became the goods depot for the London and South Western Railway Company and Waterloo became one of the biggest passenger stations in the country. The main LSWR engineering works was transferred from Nine Elms to Eastleigh by 1909. Sir Herbert Walker was General Manager of the London and South Western Railway Company from 1912, when he instituted the third-rail electrification programme, and was appointed General Manager of the Southern Railway Company in 1923, until his retirement in 1937. There were a number of notable engineers attached to the LSWR including John Viret Gooch, brother of Sir Daniel Gooch who was Locomotive Superintendent from 1841 to 1850. Dugald Drummond was the Chief Mechanical Engineer from 1895 to 1912; he facilitated moving the Works from Nine Elms to Eastleigh and their subsequent expansion and modernisation. Alfred Szlumper, appointed Chief Engineer in 1914 was key to the expansion and redesign of Waterloo Station, completed by 1922 The LSWR began electrification of its suburban lines from Waterloo in 1913. The first section was opened in 1915, and almost all the other planned routes were opened before the First World War curtailed the project. The London and South Western Railway Company was very prominent in handling all types of traffic during the First World War, due to it’s position serving London and the ports on the South Coast. The LSWR also had an ocean-going port at Southampton, which was a growing rival to Liverpool and Glasgow for transatlantic travel. The purchase of Southampton docks by the LSWR in 1892, alongside their expansion and modernisation, was organised by Sir Charles Scotter, the General Manager from 1885 to 1898. Scotter then became Chairmen of the Company from 1904 to 1910. The LSWR also ran steamer services from Southampton to the Channel Islands and France. In 1923, the LSWR was grouped into the Southern Railway Company by the Railways Act of 1921.