In the early 20th century the District and Metropolitan railways needed to electrify and cooperation between the two companies would be needed because of the shared ownership of the inner circle and a joint committee recommended the used of an AC system. The District, needing to raise the finance necessary, found an investor in the American Charles Yerkes. However, he favoured a DC system similar to that in use on the City & South London Railway and Central London Railway. The Metropolitan Railway protested about the change of plan, but after arbitration by the Board of Trade the DC system was adopted. Yerkes soon had control of the District Railway and established the Underground Electric Railways Company of London (UERL) in 1902 to complete and operate three tube lines, the Baker Street and Waterloo Railway (Bakerloo), the Charing Cross, Euston and Hampstead Railway (Hampstead) and the Great Northern, Piccadilly and Brompton Railway, (Piccadilly), which all opened between 1906 and 1907. When the 'Bakerloo' was so named in July 1906, it was called an undignified "gutter title" by The Railway Magazine. By 1907 the District and Metropolitan Railways had electrified the underground sections of their lines. A joint marketing agreement between most of the companies in the early years of the 20th century included maps, joint publicity, through ticketing and UNDERGROUND signs outside stations in Central London. The Bakerloo line was extended north to Queen's Park to join a new electric line from Euston to Watford, but the start of World War I in 1914 delayed construction and trains reached Watford Junction in 1917. An extension of the Central line east to Ealing was also delayed by the war and completed in 1920. People used the tube stations as shelters during air raids in 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 Metropolitan promoted housing estates near the railway with the "Metro-land" brand and nine housing estates were built near stations on the line and electrification was extended north from Harrow to Rickmansworth and a short branch opened from Rickmansworth to Watford in 1925, and from Wembley Park to Stanmore in 1932. The Piccadilly line was extended north to Cockfosters and took over District line branches to Harrow (later Uxbridge) and Hounslow.