Works to upgrade the existing LNER lines and construction on the new line to Bushey Heath began in the late 1930s but were halted by the outbreak of the Second World War. Additional platforms were started at Edgware and the LNER station and branch line was closed to passenger traffic in 1939 in preparation for the improvements. On the new extension, some earthworks and tunnelling had been undertaken and some structures had been constructed but no further work was done during the war.