The railway system of Great Britain, the principal territory of the United Kingdom, is the oldest in the world. The system was originally built as a patchwork of local rail links operated by small private railway companies. These isolated links developed during the railway boom of the 1840s into a national network, although still run by dozens of competing companies. Over the course of the 19th and early 20th centuries, these amalgamated or were bought by competitors until only a handful of larger companies remained. The entire network was brought under government control during the First World War and a number of advantages of amalgamation and planning were revealed. However, the government resisted calls for the nationalisation of the network. In 1923, almost all the remaining companies were grouped into the "big four", the Great Western Railway, the London and North Eastern Railway, the London, Midland and Scottish Railway and the Southern Railway. The "Big Four" were joint-stock public companies and they continued to run the railway system until 31 December 1947.
From the start of 1948, the "big four" were nationalised to form British Railways. Though there were few initial changes to the service, usage increased and the network became profitable. Declining passenger numbers and financial losses in the late 1950s and early 1960s prompted the closure of many branch and main lines, and small stations, under the Beeching Axe. Passenger services experienced a renaissance with the introduction of high-speed inter-city trains in the 1970s. The 1980s saw severe cuts in government funding and above-inflation increases in fares and the service became more cost-effective. Railway operations were privatised during 1994-1997. Ownership of the track and infrastructure passed to Railtrack, whilst passenger operations were franchised to individual private sector operators (originally there were 25 franchises) and the freight services sold outright. Passenger levels have since increased to above the level they had been at in the late-1940s. The Hatfield accident set in motion the series of events that resulted in the ultimate collapse of Railtrack and its replacement with Network Rail, a state-owned, not-for-dividend company.