The "Big Four" railway companies in Britain were created following the Railways Act 1921, which mandated the "grouping" of nearly 120 smaller companies into four large regional entities to improve efficiency after World War I. These companies, which operated from January 1, 1923, until nationalization in 1948, were the London, Midland and Scottish Railway (LMS), the London and North Eastern Railway (LNER), the Great Western Railway (GWR), and the Southern Railway (SR). The LMS was the largest, serving the industrial heartlands and Scotland; the LNER was famous for high-speed prestige trains like the Mallard; the GWR was the only one to retain its pre-grouping name and "Brunelian" identity; and the SR pioneered large-scale third-rail electrification. In 2026, their legacy remains visible in the architecture of major London termini and the colorful liveries preserved on heritage lines across the UK.