The tunnel linings were formed from cast iron segments  inch (2.22 cm) thick, which locked together to form a ring with an internal diameter of . Once a ring was completed, grout was injected through holes in the segments to fill any voids between the outside edge of the ring and the excavated ground beyond, reducing subsidence. By November 1899 the northbound tunnel reached Trafalgar Square and work on some of the station sites was started, but the collapse of the L&GFC in 1900 led to works gradually coming to a halt. When the UERL was constituted in April 1902, 50 per cent of the tunnelling and 25 per cent of the station work was completed. With funds in place, work restarted and proceeded at a rate of per week, so that by February 1904 virtually all of the tunnels and underground parts of the stations between Elephant & Castle and Marylebone were complete and works on the station buildings were under way. The additional stations were incorporated as work continued elsewhere and Oxford Circus station was altered below ground following a Board of Trade inspection; at the end of 1905, the first test trains began running. Although the BS&WR had permission to continue to Paddington, no work was undertaken beyond Edgware Road.
