Just four large companies remained, nicknamed the 'Big Four': the LMS (London, Midland and Scottish Railway); the LNER (London and North Eastern Railway); the SR (Southern Railway); and the GWR (Great Western Railway).

The term "Big Four" historically refers to the four largest railway companies in the United Kingdom that operated between the 1923 grouping and nationalization in 1948. These companies 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, known for its extensive network and heavy freight. The LNER was famous for its high-speed prestige trains like the Flying Scotsman. The GWR was unique for its consistent branding and "broad gauge" heritage, while the Southern Railway focused heavily on passenger electrification in the south of England. In a modern US context, the "Big Four" often refers to the major Class I railroads: BNSF Railway, Union Pacific (UP), CSX Transportation, and Norfolk Southern (NS), which dominate the North American freight landscape.