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London Passenger Transport Board era

In 1933, most of London's underground railways, tramway and bus services were merged to form the London Passenger Transport Board, which used the London Transport brand. The Waterloo & City Railway, which was by then in the ownership of the main line Southern Railway, remained with its existing owners. In the same year that the London Passenger Transport Board was formed, Harry Beck 's diagrammatic tube map first appeared.

In the following years, the outlying lines of the former Metropolitan Railway closed, the Brill Tramway in 1935, and the line from Quainton Road to Verney Junction in 1936. The 1935–40 New Works Programme included the extension of the Central and Northern lines and the Bakerloo line to take over the Metropolitan's Stanmore branch. The Second World War suspended these plans after the Bakerloo line had reached Stanmore and the Northern line High Barnet and Mill Hill East in 1941. Following bombing in 1940, passenger services over the West London line were suspended, leaving Olympia exhibition centre without a railway service until a District line shuttle from Earl's Court began after the war. After work restarted on the Central line extensions in east and west London, these were completed in 1949.

During the war many tube stations were used as air-raid shelters. They were not always a guarantee of safety however; on 11 January 1941 during the London Blitz, a bomb penetrated the booking hall of Bank Station, the blast from which killed 111 people, many of whom were sleeping in passageways and on platforms. On 3 March 1943, a test of the air-raid warning sirens, together with the firing of a new type of anti-aircraft rocket, resulted in a crush of people attempting to take shelter in Bethnal Green Underground station. A total of 173 people, including 62 children, died, making this both the worst civilian disaster in Britain during the Second World War, and the largest loss of life in a single incident on the London Underground network.