The history of the London Underground began in the 19th century with the construction of the Metropolitan Railway, the world's first underground railway. The Metropolitan Railway, which opened in 1863 using gas-lit wooden carriages hauled by steam locomotives, worked with the District Railway to complete London's Circle line in 1884. Both railways expanded, the Metropolitan eventually extending as far as in Buckinghamshire, more than from Baker Street and the centre of London. The first deep-level tube line, the City and South London Railway, opened in 1890 with electric trains. This was followed by the Waterloo & City Railway in 1898, the Central London Railway in 1900, and the Great Northern and City Railway in 1904. The Underground Electric Railways Company of London (UERL) was established in 1902 to fund the electrification of the District Railway and to complete and operate three tube lines, the Baker Street and Waterloo Railway, the Charing Cross, Euston and Hampstead Railway and the Great Northern, Piccadilly and Brompton Railway, which opened in 1906–07. By 1907 the District and Metropolitan Railways had electrified the underground sections of their lines. Under a joint marketing agreement between most of the companies in the early years of the 20th century, UNDERGROUND signs appeared outside stations in central London. World War I delayed extensions of the Bakerloo and Central London Railways, and people used the tube stations as shelters during Zeppelin air raids by June 1915. After the war, government-backed financial guarantees were used to expand the network, and the tunnels of the City and South London and Charing Cross, Euston and Hampstead Railways were linked at Euston and Kennington, although the combined service was not named the Northern line until later. The Piccadilly line was extended north to Cockfosters and took over District line branches to Harrow (later Uxbridge) and Hounslow. In 1933, the underground railways and all London area tram and bus operators were merged into the London Passenger Transport Board (LPTB). The outlying branches of the Metropolitan were closed; various upgrades were planned. The Bakerloo line's extension to take over the Metropolitan's Stanmore branch, and extensions of the Central and Northern lines, formed part of the 1930s New Works Programme. The outbreak of World War II in 1939 halted or interrupted some of this work, and many tube stations were used as air-raid shelters. The LPTB was nationalised in 1948, and the reconstruction of the mainline railways