The Big Four was a name used to describe the four largest railway companies in the United Kingdom in the period 1923–47. The name was coined by The Railway Magazine in its issue of February 1923: "The Big Four of the New Railway Era".
The Big Four were:
The companies were formed as a result of the Railways Act 1921, in a process known as "The Grouping" (of the railways), which came into effect on 1 January 1923.
On 1 January 1948 the companies were nationalised to form British Railways as a result of the Transport Act 1947.
The three larger companies relied heavily on freight (especially coal) and long-distance passenger traffic. The Southern Railway, in contrast, was predominantly a passenger railway, which, despite its small size, carried more than a quarter of the UK's total passenger traffic. This was because the area covered by the railway included many of the dense commuter lines around London, as well as some of the most densely populated parts of the country. It responded to this geography by pursuing a vigorous policy of electrification.
The GWR was the only company to retain its pre-grouping identity, which it duly imposed on all that it absorbed. However, the other three found that past influences remained strong. The Southern's management remained decentralised, respecting the three distinct bundles of routes inherited from its constituents. The LMS struggled to reconcile different traditions, especially in locomotive engineering, only resolving that issue in 1932 with the appointment of Sir William Stanier from the GWR. The LNER never made a profit, a fact partly accounted for by having inherited the huge debts incurred by the Great Central Railway in building its extension to London.
Although nominally in competition, the four companies worked together on projects of significance to the railway industry as a whole.
During World War II the railway companies' managements were united, effectively becoming one company, under the direction of the Railway Executive Committee. The railways were hired by the Government from 1 January 1941, to continue for one year after the end of the war. In return, a fixed Annual Rent of £43,468,705 was payable, divided between the companies according to a set formula.