King William Street opened on 18 December 1890 and was constructed from a large masonry station tunnel, accessed from the surface by a lift shaft or spiral staircase. Two platforms were provided, one on each side of the single, central track — one for passengers entering and the other for passengers leaving the trains — a system later referred to as the Spanish solution. The station tunnel itself is situated beneath Monument Street and runs east-west across King William Street, ending beneath Arthur Street. The approach running tunnels had sharp curves and steep gradients in order to dive underneath the River Thames while remaining under public rights-of-way, in particular Swan Lane and Arthur Street. The combination of station layout and poor alignment of the running tunnels severely limited the capacity of the station and, in the years after opening, a number of initiatives were made to improve operations. In 1895, a central island platform with tracks each side was constructed to enable two trains to occupy the station at once; however, capacity remained restricted. When the line was extended northwards to Moorgate, new running tunnels on a different alignment, but still beneath Borough High Street, were constructed running from below St. George the Martyr's church, north of Borough station, to a new station at London Bridge and onwards to an alternative City station at Bank. Under the Thames, the present running tunnels of the Northern line are situated to the east of London Bridge, whereas the King William Street tunnels pass to the west of the bridge, with the southbound tunnel below the northbound as the line passes under the Thames. The station closed on 24 February 1900. The original station building was demolished in the 1930s, although the parts of the station below ground were converted for use as a public air-raid shelter during World War II. Access today is via a manhole in the basement of Regis House, a contemporary office building, where the original cast iron spiral staircase leads down to platform level. The lift shaft was infilled with concrete during the construction of the original Regis House. The original running tunnels north of Borough station remain, although when the Jubilee line extension was built in the late 1990s the old southbound tunnel was cut through as part of the construction works at London Bridge station in order to provide the lift shaft situated at the south end of the Northern line platforms. These running tunnels now serve as a ventilation shaft for the station and the openings for several adits to the old running tunnels can be seen in the roofs of the Northern line platform tunnels and in the central concourse between them. A construction shaft between London Bridge and King William Street, beneath Old Swan Wharf, now serves as a pump shaft for the disused sections of running tunnels. It is no longer possible to walk through between the two stations as the old C&SLR running tunnels have been blocked off with concrete bulkheads on both sides of the Thames. The tunnels will be further severed by upgrades to Bank station, as the construction shaft for the new Northern line tunnel, located in Arthur Street, will cut straight through the approach tunnel to King William Street station, with the station site itself linked to the new tunnel via a ventilation adit.
