The following is a list of constituents of British Railways. British Railways (BR) was formed by nationalisation on 1 January 1948 in accordance with the Transport Act 1947. It had four major constituents, but there were a number of joint lines between these, and additionally some light railways were taken into the fold. Even then, some light railways were not nationalised.

Major constituents

The major constituents (the "Big Four") were:

Joint railways

Joint railways between the four major constituents (and others) were: Joint railways between the four major constituents and others were:

Minor railways and light railways

Irish railways

Irish railways owned by the LMS (the Northern Counties Committee and Dundalk, Newry and Greenore Railway lines) were also acquired but the NCC was quickly sold on to the Ulster Transport Authority, the transport unit of the Government of Northern Ireland as a result of the Ireland Act 1949. The Dundalk, Newry and Greenore Railway closed in 1951.