 When opened in 1906, the BS & WR's line served nine stations and ran completely underground in a pair of tunnels for 5.81 kilometres (3.61 mi) between its northern terminus at Baker Street and its southern terminus at Elephant and Castle with a depot on a short spur nearby at London Road. Extensions between 1907 and 1913 took the northern end of the line to the terminus of the Great Western Railway (GWR) at Paddington. Between 1915 and 1917, it was further extended to Queen's Park, where it came to the surface and connected with the London and North Western Railway (LNWR), and to Watford; a total distance of 33.34 kilometres (20.72 mi).
