The Southern Railway (SR), was a British railway company established in the 1923 Grouping. It linked London with the Channel ports, South West England and Kent. The railway was formed by the amalgamation of several smaller railway companies, the largest of which were the London & South Western Railway (LSWR), the London, Brighton and South Coast Railway (LBSC), and the South Eastern and Chatham Railway (SECR). The construction of what was to become the Southern Railway began in 1838 with the opening of the London and Southampton Railway, which was renamed the London & South Western Railway.

The railway was noted for its astute use of public relations and a coherent management structure headed by Sir Herbert Walker. At 2,186 miles (3,518 km), the Southern Railway was the smallest of the "Big Four" railway companies, and unlike the others the majority of its revenue came from passenger working rather than freight. It created what was at that time the world's largest electrified railway system, and the first electrified InterCity route (London--Brighton). There were two Chief Mechanical Engineers; Richard Maunsell between 1923 and 1937, and Oliver Bulleid from 1937 to 1948, both of whom designed new locomotives and rolling stock to replace much of that which was inherited in 1923. The Southern Railway played a vital role in the Second World War, embarking the British Expeditionary Force, during the Dunkirk operations, and supplying Operation Overlord in 1944: because the railway was primarily a passenger network its success was an even more remarkable achievement.

The Southern Railway operated a number of famous named trains, including the Brighton Belle, the Bournemouth Belle, the Golden Arrow, and the Night Ferry (London - Paris and Brussels). The West Country services were dominated by lucrative summer holiday traffic and included named trains such as the Atlantic Coast Express and the Devon Belle. The company's best-known livery was highly distinctive: locomotives and carriages were painted in a bright Malachite green above plain black frames, with bold, bright yellow lettering. The Southern Railway was nationalised in 1948, becoming the Southern Region of British Railways.