Somerset and Dorset Railway services were jointly run by BR(S) and BR Midland Region (BR(M)) then latterly BR Western Region (BR(W)) until closure.

In 1950 the former SR and BR(S) lines in the West Country (i.e. west of Exeter) were re-assigned by the British Railways Board (BRB) to the BR Western Region BR(W). Running the services remained the responsibility of the Southern Region although the infrastructure was maintained by the Western Region. Full responsibility for these lines returned to the BR(S) in 1958. However in 1963 BR(S) lines and services in the West Country and on the main line to Exeter west of Wilton was handed over to the Western Region.  BR(W) promptly downgraded services on the former LSWR main line to the status of a secondary line and favoured their own but more circuitous and remote route via Westbury. Much of the former double track main line was singled.

On 4th January 1982 5 "business sectors" were formally created to run services although infrastructure, etc, remained the responsibility of the regions.  The majority of BR(S) services came under the control of the London and South East (Passenger) Sector.  Following the appointment of Chris Green as its Director the sector was re-branded amid much publicity as Network SouthEast on June 10th 1986.

On 29th April 1991 under the British Rail "Organisation for Quality" restructuring the regional structure was dissolved and full responsibility for all aspects of organisation and running the railway passed to the business sectors. The last vestiges of the Southern Railway effectively disappeared under this fundamental and far-reaching restructuring.