The Railways Act 1921 in the United Kingdom was a post-World War 1 measure to consolidate the fragmented railway system whilst leaving the sector in private ownership.  On the island of Great Britain, almost every common carrier railway was incorporated into the so-called "Big Four".  They were:

• London and North Eastern Railway (LNER)
• London, Midland and Scottish Railway (LMS)
• Great Western Railway (GWR)
• Southern Railway (SR)

The companies only lasted for 25 years until Nationalisation in 1948 - and part of that time was spent under government control during World War 2.

Government control of railways on the island of Ireland ended in 1921 as the Irish Free State was established.  The LMS absorbed about 40% of lines in Northern Ireland; the Great Northern Railway of Ireland operated most of the rest until being nationalised end-1958.