The London, Midland and Scottish Railway (LMS) was one of the Big Four British railway companies from 1923 to 1948. It was formed on 1 January 1923 under the Railways Act of 1921, which required the grouping of over 120 separate railways into four. The companies merged into the LMS included the London and North Western Railway, the Midland Railway, the Lancashire and Yorkshire Railway (which had previously merged with the London and North Western Railway on 1 January 1922), several Scottish railway companies (including the Caledonian Railway), and numerous other, smaller ventures.