 The linking of the GN & SR and B & PCR routes at Holborn meant that the section of the GN & SR south of Holborn became a branch from the main route . The UERL began constructing the main route in July 1902 . Progress was rapid , so that it was largely complete by the Autumn of 1906 . Construction of the branch was delayed while the London County Council carried out slum clearances to construct its new road Kingsway and the tramway subway running beneath it and while the UERL decided how the junction between the main route and the branch would be arranged at Holborn . 
