The Southern Railway (SR) was established in the 1923 Grouping, linking London with the Channel ports, South West England and Kent. It was formed by the amalgamation of several smaller railway companies, the largest of which were the the London, Brighton and South Coast Railway (LBSC),  the London & South Western Railway (LSWR) and the South Eastern and Chatham Railway (SECR).

At 3,518 km (2,186 miles) the Southern Railway was the smallest of the "Big Four" railway companies but – uniquely - the majority of its revenue came from passenger working rather than freight. It not only created the then world's largest electrified railway system, but also the first electrified inter-city route, from London to Brighton; the Brighton Belle represented a milestone in mass passenger transport and the mobility of labour.

The railway was noted for its astute use of public relations and a coherent management structure, headed by Sir Herbert Walker. Over its lifetime it employed two notable Chief Mechanical Engineers -  Richard Maunsell, between 1923 and 1937, and Oliver Bulleid, from 1937 to 1948. Both 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 – not only the Brighton Belle, but also 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, with 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.