British Railways was the national railway company from 1948 to 1997. Created under the Transport Act of 1947 that merged the 'Big Four' companies of the Southern Railway, London & North Eastern Railway, Great Western Railway, and the London Midland & Scottish Railway.
The system was initially divided into six regions, which covered the respective areas operated by the former 'Big Four' companies, with the exception of Scotland which got its own region, Scottish Region. The Southern Region covered the former Southern Railway, the Eastern Region was the southern part of the former London & North Eastern Railway, the North Eastern Region was the former northern part of the London & North Eastern Railway, the Western Region was the former Great Western Railway, and the London Midland Region was the former London Midland & Scottish Railway.
Over time the regions changed their operation areas, for example the London Tilbury and Southend line had been part of the London Midland & Scottish Railway, and therefore became part of the London Midland Region. As this line was within the territory of the Eastern Region, it was transferred to them.