The major constituents of the Southern Railway were the London and South Western Railway, the London, Brighton and South Coast Railway and the combined systems of the South Eastern Railway and the London, Chatham and Dover Railway. These plus a number of smaller concerns gave the Southern a route mileage of 2,186.
In the area south and east of London the Southern Railway was a virtual monopoly, while its lines to the South-West were largely in competition with the Great Western Railway.
Unlike the three other railway systems remaining after Grouping (the London, Midland and Scottish Railway, the London and North Eastern Railway and the Great Western, the Southern Railway was a predominantly passenger-oriented railway. The number of passengers carried by the Southern were over a quarter of the nation's total. This is partly because the area covered by the railway encompassed the most populated areas of the nation and much of the commutable zone around London.
The density of much of the Southern's trackage and traffic made it a natural candidate for electrification, and much of the South-Eastern section of the railway was electrified using a side-mounted, top-contact third rail system at 600 volts DC. Originally, only electric multiple unit cars used the electrical power, but later on a number of electric locomotives and electric/diesel hybrids were developed.
Chief Mechanical Engineers of the Southern Railway, responsible for locomotives and rolling stock, included R. E. L. Maunsell and then, from 1937, O. V. S. Bullied.