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.

Railways in Britain were nationalised under the terms of the Transport Act 1947 which came into effect on 1 January 1948. The Act created the British Transport Commission and the Railway Executive. The Act vested the business and assets of the then existing railway companies in the British Transport Commission. The Railway Executive, a corporate body subordinate to the British Transport Commission, was created to manage and operate the railways. It divided them into six geographical regions, largely based on the areas served by the pre-nationalisation railway companies, one of which was Southern Region. It comprised the railway operations in England and Wales of the former Southern Railway Company. Although several lines previously belonging to former railway companies were transferred to it, notably sections of the former Great Western Railway lines to Weymouth, the Midland & South West Junction between Grafton and Burbage, the Didcot Newbury & Southampton between Newbury and Windsor, and the Reading – Basingstoke and Westbury – Salisbury lines, in an attempt to remove “penetrating lines”, Southern Region kept the line from Exeter to Barnstaple and Ilfracombe, which ran through Western Region territory. This line was transferred to Western Region in 1963. Some of its commuter services and lines were transferred to London Underground. Although several branch lines closed during its existence, Southern Region, with its heavy-used passenger services, did not experience closures on the scale of other regions. Southern Region served south London, southern England and the south coast as far west as Exeter. There were three aspects to its services: those in the London commuter areas of Kent, Sussex and Surrey, its long-distance services to the West Country from Waterloo station and its international service by rail ferry jointly with SNCF (French state railways). Much of the commuter network had been electrified by Southern Region’s predecessor companies on the third-rail 660 volt direct current system. Although British Railways’ policy was to electrify on the overhead 25000 volt alternating current system, Southern Region extended its third-rail electrification in the 1960s and 1970s. Between 1948 and 1953 the regional manager was responsible to the Railway Executive for day to day operations in his region. After the Railway Executive was abolished in 1953, he reported to the British Transport Commission. In 1963, the British Transport Commission itself was abolished and replaced by British Railways Board. Between 1963 and 1968 Southern Region was a statutory board in accordance with the provisions of the Transport Act 1962, subordinate to and reporting to British Railways Board. It ceased to be a statutory board in 1968, following reorganisation of the railways’ business along functional lines. The name survived until 1992 when the railways were privatised.