During the first year or so of British Railways, some of the simpler and more obvious inter-regional transfers of outlying sections were effected, such as those of the London, Tilbury & Southend Railway from the London Midland Region to the Eastern Region; the South Wales lines of the former L.M.S.R. to the Western Region; the Carlisle-Silloth branch (an L.N.E.R. legacy of a North British "border raid") to the London Midland, and so on.
Source: wiktionary