The railway was noted for its astute use of public relations and a coherent management structure headed by Sir Herbert Walker. At , the Southern Railway was the smallest of the "Big Four" railway companies and, unlike the others, the majority of its revenue came from passenger traffic rather than freight. It created what was at that time the world's largest electrified main line railway system and the first electrified inter-city 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.