The Big Four British railway companies operated most of the trains in Great Britain from 1923 to 1947. They were created by merging (known as grouping) a large number of smaller railway companies following the passing of the Railways Act 1921 by the British government.
The Big Four were:
The Great Western Railway existed before 1923 and kept its original name. In 1923, it took over most of the railways in Wales.
The main constituents of the London, Midland and Scottish Railway were:
The main constituents of the London and North Eastern Railway were:
The main constituents of the Southern Railway were:
On 1 January 1948, the Big Four were nationalized to form British Railways, which later became British Rail.