The London Underground is a metro system serving a large part of Greater London and neighbouring areas of Essex Hertfordshire and Buckinghamshire in the UK. It is the world's oldest underground railway. It was also the first underground railway to operate electric trains. It is usually referred to as the Underground or the Tube—the latter deriving from the shape of the system's deep-bore tunnels—although about 55% of the network is above ground.
The earlier lines of the present London Underground network which were built by various private companies became part of an integrated transport system (which excluded the main line railways) in 1933 with the creation of the London Passenger Transport Board (LPTB) more commonly known by its shortened name: "London Transport". The underground network became a single entity when London Underground Limited (LUL) was formed by the UK government in 1985. Since 2003 LUL has been a wholly owned subsidiary of Transport for London (TfL) the statutory corporation responsible for most aspects of the transport system in Greater London which is run by a board and a commissioner appointed by the Mayor of London.
The Underground has 270 stations and approximately 400 km (250 miles) of track making it the longest metro system in the world by route length  and one of the most served in terms of stations. In 2007 over one billion passenger journeys were recorded.
The tube map with its schematic non-geographical layout and colour-coded lines is considered a design classic and many other transport maps worldwide have been influenced by it.
Main article: History of the London Underground
Railway construction in the United Kingdom began in the early 19th century. By 1854 six separate railway terminals had been built just outside the centre of London: London Bridge Euston Paddington King's Cross Bishopsgate and Waterloo. At this point only Fenchurch Street Station was located in the actual City of London. Traffic congestion in the city and the surrounding areas had increased significantly in this period partly due to the need for rail travellers to complete their journeys into the city centre by road. The idea of building an underground railway to link the City of London with the mainline terminals had first been proposed in the 1830s but it was not until the 1850s that the idea was taken seriously as a solution to the traffic congestion problems.
The first underground railways
In 1854 an Act of Parliament was passed approving the construction of an underground railway between Paddington Station and Farringdon Street via King's Cross which was to be called the Metropolitan Railway. The Great Western Railway (GWR) gave financial backing to the project when it was agreed that a junction would be built linking the underground railway with their mainline terminus at Paddington. GWR also agreed to design special trains for the new subterranean railway.
Construction was delayed for several years due to a shortage of funds. The fact that this project got under way at all was largely due to the lobbying of Charles Pearson who was Solicitor to the City of London Corporation at the time. Pearson had supported the idea of an underground railway in London for several years. He advocated plans for the demolition of the unhygienic slums which would be replaced by new accommodation for their inhabitants in the suburbs with the new railway providing transportation to their places of work in the city centre. Although he was never directly involved in the running of the Metropolitan Railway he is widely credited as being one of the first true visionaries behind the concept of underground railways. And in 1859 it was Pearson who persuaded the City of London Corporation to help fund the scheme. Work finally began in February 1860 under the guidance of chief engineer John Fowler. Pearson died before the work was completed.
The Metropolitan Railway opened on 10 January 1863. Within a few months of opening it was carrying over 26 000 passengers a day. The Hammersmith and City Railway was opened on 13 June 1864 between Hammersmith and Paddington. Services were initially operated by GWR between Hammersmith and Farringdon Street. By April 1865 the Metropolitan had taken over the service. On 23 December 1865 the Metropolitan's eastern extension to Moorgate Street opened. Later in the decade other branches were opened to Swiss Cottage South Kensington and Addison Road Kensington (now known as Kensington Olympia). The railway had initially been dual gauge allowing for the use of GWR's signature broad gauge rolling stock and the more widely used standard gauge stock. Disagreements with GWR had forced the Metropolitan to switch to standard gauge in 1863 after GWR withdrew all its stock from the railway. These differences were later patched up however broad gauge was totally withdrawn from the railway in March 1869.
On 24 December 1868 the Metropolitan District Railway began operating services between South Kensington and Westminster using Metropolitan Railway trains and carriages. The company which soon became known as "the District" was first incorporated in 1864 to complete an Inner Circle railway around London in conjunction with the Metropolitan. This was part of a plan to build both an Inner Circle line and Outer Circle line around London.
A fierce rivalry soon developed between the District and the Metropolitan. This severely delayed the completion of the Inner Circle project as the two companies competed to build far more financially lucrative railways in the suburbs of London. The London and North Western Railway (LNWR) began running their Outer Circle service from Broad Street via Willesden Junction Addison Road and Earl's Court to Mansion House in 1872. The Inner Circle was not completed until 1884 with the Metropolitan and the District jointly running services. In the meantime the District had finished its route between West Brompton and Blackfriars in 1870 with an interchange with the Metropolitan at South Kensington. In 1877 it began running its own services from Hammersmith to Richmond on a line which had originally opened by the London & South Western Railway (LSWR) in 1869. The District then opened a new line from Turnham Green to Ealing in 1879 and extended its West Brompton branch to Fulham in 1880. Over the same decade the Metropolitan was extended to Harrow-on-the-Hill station in the north-west.
The early tunnels were dug mainly using cut-and-cover construction methods. This caused widespread disruption and required the demolition of several properties on the surface. The first trains were steam-hauled which required effective ventilation to the surface. Ventilation shafts at various points on the route allowed the engines to expel steam and bring fresh air into the tunnels. One such vent is at Leinster Gardens W2. In order to preserve the visual characteristics in what is still a well-to-do street a five-foot-thick (1.5 m) concrete façade was constructed to resemble a genuine house frontage.
On 7 December 1869 the London Brighton and South Coast Railway (LB&SCR) started operating a service between Wapping and New Cross Gate on the East London Railway (ELR) using the Thames Tunnel designed by Marc Brunel who designed the revolutionary tunnelling shield method which made its construction not only possible but safer and completed by his son Isambard Kingdom Brunel. This had opened in 1843 as a pedestrian tunnel but in 1865 it was purchased by the ELR (a consortium of six railway companies: the Great Eastern Railway (GER); London Brighton and South Coast Railway (LB&SCR); London Chatham and Dover Railway (LCDR); South Eastern Railway (SER); Metropolitan Railway; and the Metropolitan District Railway) and converted into a railway tunnel. In 1884 the District and the Metropolitan began to operate services on the line.
By the end of the 1880s underground railways reached Chesham on the Metropolitan Hounslow Wimbledon and Whitechapel on the District and New Cross on the East London Railway. By the end of the 19th century the Metropolitan had extended its lines far outside of London to Aylesbury Verney Junction and Brill creating new suburbs along the route—later publicised by the company as Metro-land. Right up until the 1930s the company maintained ambitions to be considered as a main line rather than an urban railway.
The first tube lines
Following advances in the use of tunnelling shields electric traction and deep-level tunnel designs later railways were built even further underground. This caused much less disruption at ground level and it was therefore cheaper and preferable to the cut-and-cover construction method.
The City & South London Railway (C&SLR now part of the Northern Line) opened in 1890 between Stockwell and the now closed original terminus at King William Street. It was the first "deep-level" electrically operated railway in the world. By 1900 it had been extended at both ends to Clapham Common in the south and Moorgate Street (via a diversion) in the north. The second such railway the Waterloo and City Railway opened in 1898. It was built and run by the London and South Western Railway.
On 30 July 1900 the Central London Railway (now known as the Central Line) was opened operating services from Bank to Shepherd's Bush. It was nicknamed the "Twopenny Tube" for its flat fare and cylindrical tunnels; the "tube" nickname was eventually transferred to the Underground system as a whole. An interchange with the C&SLR was provided at Bank. Construction had also begun in August 1898 on the Baker Street & Waterloo Railway. However work on this railway came to a halt 18 months after it began when funds ran out.
In the early 20th century the presence of six independent operators running different Underground lines caused passengers substantial inconvenience; in many places passengers had to walk some distance above ground to change between lines. The costs associated with running such a system were also heavy and as a result many companies looked to financiers who could give them the money they needed to expand into the lucrative suburbs as well as electrify the earlier steam operated lines. The most prominent of these was Charles Yerkes an American tycoon who secured the right to build the Charing Cross Euston and Hampstead Railway (CCE&HR) on 1 October 1900. In March 1901 he effectively took control of the District and this enabled him to form the Metropolitan District Electric Traction Company (MDET) on 15 July. Through this he acquired the Great Northern & Strand Railway and the Brompton & Piccadilly Circus Railway in September 1901 the construction of which had already been authorised by Parliament together with the moribund Baker Street & Waterloo Railway in March 1902. On 9 April the MDET evolved into the Underground Electric Railways of London Company Ltd (UERL). The UERL also owned three tramway companies and went on to buy the London General Omnibus Company creating an organisation colloquially known as "the Combine" which went on to dominate underground railway construction in London until the 1930s.
With the financial backing of Yerkes the District opened its South Harrow branch in 1903 and completed its link to the Metropolitan's Uxbridge branch at Rayners Lane in 1904—although services to Uxbridge on the District did not begin until 1910 due to yet another disagreement with the Metropolitan. By the end of 1905 all District Railway and Inner Circle services were run by electric trains.
The Baker Street & Waterloo Railway opened in 1906 soon branding itself the Bakerloo and by 1907 it had been extended to Edgware Road in the north and Elephant & Castle in the south. The newly named Great Northern Piccadilly and Brompton Railway combining the two projects acquired by MDET in September 1901 also opened in 1906. With tunnels at an impressive depth of 200 feet below the surface it ran from Finsbury Park to Hammersmith; a single station branch to Strand (later renamed Aldwych) was added in 1907. In the same year the CCE&HR opened from Charing Cross to Camden Town with two northward branches one to Golders Green and one to Highgate (now Archway).
Independent ventures did continue in the early part of the 20th century. The independent Great Northern & City Railway opened in 1904 between Finsbury Park and Moorgate. It was the only tube line of sufficient diameter to be capable of handling main line stock and it was originally intended to be part of a main line railway. However money soon ran out and the route remained separate from the main line network until the 1970s. The C&SLR was also extended northwards to Euston by 1907.
In early 1908 in an effort to increase passenger numbers the underground railway operators agreed to promote their services jointly as "the Underground" publishing new adverts and creating a free publicity map of the network for the purpose. The map featured a key labelling the Bakerloo Railway the Central London Railway the City & South London Railway the District Railway the Great Northern & City Railway the Hampstead Railway (the shortened name of the CCE&HR) the Metropolitan Railway and the Piccadilly Railway. Some other railways appeared on the map but with less prominence than the aforementioned lines. These included part of the ELR (although the map wasn't big enough to fit in the whole line) and the Waterloo and City Railway. As the latter was owned by a main line railway company it wasn't included in this early phase of integration. As part of the process "The Underground" name appeared on stations for the first time and electric ticket-issuing machines were also introduced. This was followed in 1913 by the first appearance of the famous circle and horizontal bar symbol known as "the roundel"  designed by Edward Johnston.
On 1 January 1913 the UERL absorbed two other independent tube lines the C&SLR and the Central London Railway. As the Combine expanded only the Metropolitan stayed away from this process of integration retaining its ambition to be considered as a main line railway. Proposals were put forward for a merger between the two companies in 1913 but the plan was rejected by the Metropolitan. In the same year the company asserted its independence by buying out the cash strapped Great Northern and City Railway. It also sought a character of its own. The Metropolitan Surplus Lands Committee had been formed in 1887 to develop accommodation alongside the railway and in 1919 Metropolitan Railway Country Estates Ltd. was founded to capitalise on the post-World War One demand for housing. This ensured that the Metropolitan would retain an independent image until the creation of London Transport in 1933.
The Metropolitan also sought to electrify its lines. The District and the Metropolitan had agreed to use the low voltage dc system for the Inner Circle comprising two electric rails to power the trains back in 1901. At the start of 1905 electric trains began to work the Uxbridge branch and from 1 November 1906 electric locomotives took trains as far as Wembley Park where steam trains took over.