The City & South London Railway (C&SLR), known as City of London & Southwark Subway for a time prior to its opening, is considered to be the first real deep-level "tube" railway in the world. Today, the line forms part of the Bank branch of the Northern Line of the London Underground network.
Built under the supervision of James Henry Greathead (using some of the techniques that he had used on the earlier Tower Subway project) it was opened officially between Stockwell and King William Street (near Monument) on 4 November 1890. The public opening followed over a month later, on 18 December 1890.
Passengers were carried in carriages with longitudinal seating and sliding doors at the ends, leading onto a platform from which they could board and alight. Gatemen rode on the platforms and were responsible for opening and closing the lattice gates that enclosed the platforms. The carriages soon became known as "padded cells", due to their claustrophobic interior and to the high-backed seating. As it was reasoned that there was nothing to look at in the tunnels, there were no true windows, only very small frosted glass panels high above the seats. Because stations were not visible from inside a carriage, the gatemen were required to announce the names of the stations. One of these carriages can today be seen in London's Transport Museum. The trains were originally composed of three carriages hauled by small electric locomotives. Although train length was gradually increased, locomotive haulage remained in use until 1923.
The company constructed a depot on the surface at Stockwell with access to the railway via a steep incline tunnel (later replaced by a lift). In practice, the incline was little used, most rolling stock and locomotives going to the surface only for major maintenance. The depot area also contained a generating station to produce power for the railway. The size of the generators and the unreliability of electricity at that time resulted in the stations being illuminated originally by gas. Access to the stations was by spiral staircases and lifts, the hydraulic power for the lifts also being produced at Stockwell depot.
In 1900 the C&SLR was extended to the north following City Road to Islington (via Moorgate, Old Street, City Road (closed today) and Angel), and south to Clapham Common. The former northern terminus at King William Street was problematic due to its lack of capacity, a difficult approach curve and its alignment making it impossible to extend the line from this point. With the 1900 extensions, new tunnels were opened north of Borough station allowing King William Street station to be closed and replaced by Bank station. King's Cross and Euston were reached in 1907, where the C&SLR met (but did not join) the Charing Cross, Euston & Hampstead Railway (CCE&HR).
The two companies merged in 1913, and following a series of improvement and extension projects in the early 1920s the combined line developed to form the majority of what is now the Northern Line by 1926, although the name did not come into use until the 1930s. The tube tunnels that formed the railway had been built to three different diameters (10' 2" from King William Street to Elephant & Castle, 10' 6" from Elephant & Castle to Stockwell and 11' 6" for the remainder of the railway) and it therefore became necessary to enlarge the tunnels to create some form of network standard size that would allow the through running of trains. Although different methods were used to enlarge the tunnels on the southern and northern sections of the line, both of these involved expanding the diameter by adding adding new pieces to the segments that made up the tunnel lining. This meant that, although the tunnels were enlarged to notional 11' 8.25", the partial reuse of existing linings meant that the tunnels were (and still are) of an irregular shape with no measurement that can definitively be quoted as an overall diameter.
Much of the reconstruction work was carried out while a daytime passenger service still operated on the line south of Moorgate but, following a collapse in the roof of a tunnel, traffic was stopped late in 1923 while the reconstruction was completed.