The Southern Railway was still comparatively profit-making despite World War II, thanks to its extensive third rail DC electrification and the intensive service patterns this allowed for.

The Transport Act 1947 provided for the nationalisation of all heavy rail systems in the UK to allow for this investment and, in theory, to improve the rights of railway workers.

There was also an unelectrified service to parts of Devon and Cornwall, deep in what was largely Western Region territory, known colloquially as "The Withered Arm".

The Beeching Axe severely cut route mileages of most regions but the Southern escaped major losses in the London area due to high passenger numbers on its frequent suburban services.

The lines in Devon and Cornwall were reclassified to the Western Region and the Southern's luxury trains, including the Atlantic Coast Express and the Brighton Belle, ceased in the 1960s and 70s.

At the same time, Holborn Viaduct in central London closed in 1990, replaced with City Thameslink occupying the same site at an underground level.

To that end, rolling stock on the London to Dover via Ashford services was refurbished and heavier rails were laid to allow for longer trains and increased freight.

Until 1980 the Southern Region operated the Night Ferry sleeper train (jointly with SNCF) from London Victoria to Paris and Brussels.

The Southern Railway had adopted a plan to convert all lines east of Portsmouth to third rail electric traction in November 1946, to be completed by 1955.

The surviving line between Ryde and Shanklin was therefore electrified in March 1967 using converted stock originally built for London Electric Railway in 1921.

A fleet of diesel-electric multiple units, also known by enthusiasts as "Thumpers" because of their distinctive engines, ran on non- or partly-electrified routes.

Isle of Wight railway lines used elderly steam engines cascaded from the mainland for many years, but in 1966 the Southern Region acquired some redundant "Standard" tube stock from London Transport that varied build dates between 1923 and 1934.

Few Mark 2 multiple units were built but some driving trailers were introduced for Gatwick Airport express services, then operated by the InterCity brand.

British Rail built a fleet of electric units to operate Bournemouth services from Waterloo in the 1980s, with Mark 3 bodies and plug doors.

These Class 442 (5-WES) units later transferred to the Brighton Main Line in 2008 on Gatwick Express services from Victoria, run by the Southern franchise, before returning to South Western Railway in 2019.

New safety regulations prohibiting the use of trains with slam doors (unless equipped with secondary or central locking) were not introduced until the last examples could be withdrawn.

Exceptionally, some slam door units were allowed to stay in service for another couple of years by special derogation on the Lymington Pier "heritage" branch before they retired permanently in 2010.

A lack of investment and focus on infrastructure maintenance and collision safeguards across institutions led to a marginal improvement in the net rate of fatal incidents compared to the same (non-British Rail) period before nationalisation, which reduced potential passengers' confidence.