Metropolitan line
The 1930s was a period of rapid growth for the north-western suburbs of London, and LPTB developed ambitious plans to simplify the Metropolitan line and expand capacity. Several stations on the Uxbridge branch were rebuilt, replacing temporary wooden buildings with modernist designs and giving Uxbridge station a new site in the town centre. A major bottleneck in the line, the double-track tunnel from Baker Street to Finchley Road, was bypassed by boring two tube tunnels underneath the Metropolitan tunnels, transferring slow services and the Stanmore branch to the Bakerloo line. (This route was transferred to the Jubilee line in 1979.) In 1936 the line was extended east from  to  along the tracks of the District line. London Transport inherited a number of incompatible electric multiple units from the railway, including the 1927–33 multiple-unit compartment stock used on routes to Watford and Rickmansworth, and these were refurbished to form a uniform fleet and designated London Underground T Stock. In the 1950s, F Stock trains, with sliding doors under the control of the guard, were transferred from the District line; these mainly worked the semi-fast Harrow and Uxbridge services.