The United Kingdom has two major rail networks: the network in Great Britain, a standard gauge network; and the Northern Ireland network, a 5 ft 3 in (1,600 mm) gauge network (which, together with the Republic of Ireland network forms a single, unified network in Ireland).

The railway network in Great Britain consists of approximately 10,000 miles of track and serves around 2,500 stations. The railway infrastructure is owned and operated by Network Rail while passenger services and all but 17 stations are operated by a total of 26 privately owned train operating companies (Network Rail directly operate the remaining 17 principal stations). The Irish network is naturally much smaller, with just 300 miles of track in Northern Ireland and around 1,400 miles of track in the Republic of Ireland, less than half of the original total of 3,600 miles of track. There are also 1,200 miles of private 3 ft (914 mm) gauge narrow gauge railways used for transporting peat by Bord na Móna, a company of the Irish government.

In 2005/2006 there were over 1 billion passenger journeys in Great Britain, the largest number since 1959, and during 2005/6 Network Rail will have spent approximately £5.1 billion on the routine maintenance and upgrading of the network. Network Rail continues to spend the equivalent of £14 million every day on maintenance and upgrading of the network.

In Ireland, the rail network has arguably suffered from much more serious under-investment than its mainland counterpart and passenger numbers are often negligible on some routes, however the two railway companies on the island have recently spent considerable sums upgrading track and rolling stock

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 (see railway mania). 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 main branch lines and small stations in 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 which resulted in the ultimate collapse of Railtrack and its replacement with Network Rail, a state-owned, not-for-dividend company.

From the start of 1948, the railways were nationalised to form British Railways (latterly "British Rail") under the control of the British Transport Commission.Though there were few initial changes to the service, usage increased and the network became profitable. Regeneration of track and stations was completed by 1954. In the same year, changes to the British Transport Commission, including the privatisation of road haulage, ended the coordination of transport in the UK. Rail revenue fell and in 1955 the network again ceased to be profitable. The mid-1950s saw the hasty introduction of diesel and electric rolling stock to replace steam in a modernisation plan costing many millions of pounds, but the expected transfer back from road to rail did not occur and losses began to mount. This failure to make the railways more profitable through investment lead governments of all political persuasions to restrict rail investment to a drip feed and seek economies through cutbacks.

The desire for profitability led to a major reduction in the network during the mid-1960s. Dr. Richard Beeching was given the task by the government of re-organising the railways ("the Beeching Axe"). This policy resulted in many branch lines and secondary routes being closed because they were deemed uneconomic. The closure of stations serving rural communities removed much feeder traffic from main line passenger services. The closure of many freight depots that had been used by larger industries such as coal and iron led to much freight transferring to road haulage. The closures were extremely unpopular with the general public at that time, and remain so today.

Passenger levels decreased steadily from the late fifties to late seventies. Passenger services then experienced a renaissance with the introduction of the high-speed Intercity 125 trains in the late 1970s and early 1980s. The 1980s saw severe cuts in government funding and above-inflation increases in fares, and the service became more cost-effective.

Between 1994 and 1997, British Rail was privatised. Ownership of the track and infrastructure passed to Railtrack; passenger operations were franchised to individual private sector operators (originally there were 25 franchises); and the freight services sold outright (six companies were set up, but five of these were sold to the same buyer). The Conservative government under John Major said that privatisation would see an improvement in passenger services. Passenger levels have since increased to above the level they had been at in the late 1950s.