Which British law grouped most railways into the Big Four companies in 1923?

The British law that grouped most railways into the Big Four companies in 1923 was the Railways Act 1921.

The act provided the legal framework for railway grouping, a major consolidation intended to reduce competition and rationalise Britain’s fragmented railway system. The grouping took effect on 1 January 1923, creating four dominant companies: the Great Western Railway, London, Midland and Scottish Railway, London and North Eastern Railway, and Southern Railway.

The Big Four were not created by the later Transport Act 1947. That act nationalised the mainline railways and brought them together as British Railways from 1948. The earlier Railways Act 1921 therefore marks the transition from numerous private companies to the large regional groupings of the interwar period.

The four companies differed significantly. The Southern Railway was strongly associated with passenger traffic and electrification, while the LMS, LNER and GWR handled substantial freight as well as long-distance passenger services. The Big Four lasted until nationalisation after the Second World War.