The City and South London Railway (C&SLR) was the first successful deep-level underground "tube" railway in the world, and the first major railway to use electric traction. The railway was originally intended for cable-hauled trains, but owing to the bankruptcy of the cable contractor during construction, a system of electric traction using electric locomotives – an experimental technology at the time – was chosen instead.
When opened in 1890, the line had six stations and ran for 3.2 miles (5.1 km) in a pair of tunnels between the City of London and Stockwell, passing under the River Thames. The diameter of the tunnels restricted the size of the trains, and the small carriages with their high-backed seating were nicknamed padded cells. The railway was extended several times north and south, eventually serving 22 stations over a distance of 13.5 miles (21.7 km) from Camden Town in north London to Morden in south London.
Although the C&SLR was well used, low ticket prices and the construction cost of the extensions placed a strain on the company's finances. In 1913 the C&SLR became part of the Underground Group of railways, and in the 1920s it underwent major reconstruction works before its merger with another of the Group's railways, the Charing Cross, Euston and Hampstead Railway, forming a single London Underground line called the Morden–Edgware line. In 1933, the London Passenger Transport Board was created by amalgamation of the C&SLR with the rest of the Underground Group, bus and other transport companies. This was a public authority but not nationalised until 1948 with the creation of the British Transport Commission. Today, its tunnels and stations form the Bank Branch of the Northern line from Camden Town to Kennington and the southern leg of the line from Kennington to Morden.
In November 1883, notice was given that a private bill was to be presented to Parliament for the construction of the City of London and Southwark Subway (CL&SS). The promoter of the bill, and engineer of the proposed railway, was James Henry Greathead, who had, in 1869–70, constructed the Tower Subway using the same tunnelling shield/segmented cast iron tube method proposed for the CL&SS. The railway was to run from Elephant and Castle, in Southwark, south London, under the River Thames to King William Street in the City of London. The tracks were to be in twin tunnels 10 ft 2 in (3.1 metres) in diameter, running for a distance of 1.25 miles (2.01 km).
The bill received royal assent as the City of London and Southwark Subway Act 1884 (47 & 48 Vict. c. clxvii) on 28 July 1884. Section 5 of the act stated:
In 1886, a further bill was submitted to Parliament to extend the tunnels south from Elephant and Castle to Kennington and Stockwell. This received assent on 12 July 1887 as the City of London and Southwark Subway (Kennington Extensions, &c.) Act 1887 (50 & 51 Vict. c. cv), allowing the construction of the extension to be added to the work on the original route, which had begun in 1886. The tunnels on this section were of a slightly larger diameter – 10 ft 6 in (3.2 metres) and extended the line by a further 1.75 miles (2.82 km). Before the railway opened, a further bill received assent, granting permission to continue the line south to Clapham Common. The act was given royal assent on 25 July 1890 as the City and South London Railway Act 1890 (53 & 54 Vict. c. cxiv), also effecting a change of the company's name.
Because of the small diameter of the tunnels and the difficulty of providing sufficient ventilation, steam power, which was used on London's other underground railways, would not work for a deep-level tube railway. Like Greathead's earlier Tower Subway, the CL&SS was intended to be operated by cable haulage with a static engine pulling the cable through the tunnels at a steady speed. Section 5 of the 1884 act specified that:
The traffic of the subway shall be worked by ... the system of the Patent Cable Tramway Corporation Limited or by such means other than steam locomotives as the Board of Trade may from time to time approve.
The Patent Cable Tramway Corporation owned the rights to the Hallidie cable-car system first invented and used in San Francisco in 1873; trains were attached to the cable with clamps, which would be opened and closed at stations, allowing the carriages to disconnect and reconnect without needing to stop the cable or to interfere with other trains sharing the cable. There were to be two independent endless cables, one between City station and Elephant and Castle moving at 10 mph (16 km/h), and the other between Elephant and Castle and Stockwell, where the gradient was less, at 12 mph (19 km/h). However, the additional length of tunnel permitted by the supplementary acts challenged the practicality of the cable system.
It is reported that this problem with the CL&SS contributed to the bankruptcy of the cable company in 1888. However, electric motor traction had been considered all along, and much engineering progress had been made since the tunnel's construction had begun in 1886. Hence, CL&SS chairman Charles Grey Mott decided to switch to electric traction. Other cable-operated systems using the Hallidie patents continued to be designed, such as the Glasgow Subway which opened in 1896.
The solution adopted was electrical power, provided via a third rail (now a fourth rail) beneath the train, but offset to the west of centre for clearance reasons. Although the use of electricity to power trains had been experimented with during the previous decade, and small-scale operations had been implemented, the C&SLR was the first major railway in the world to adopt it as a means of motive power. The system operated using electric locomotives built by Mather & Platt collecting a voltage of 500 volts (actually +500 volts in the northbound tunnel and −500 volts in the southbound) from the third rail and pulling several carriages. A depot and generating station were constructed at Stockwell. Owing to the limited capacity of the generators, the stations were originally illuminated by gas. The depot was on the surface, and trains requiring maintenance were initially hauled up via a ramp although, following a runaway accident, a lift was soon installed. In practice, most rolling stock and locomotives went to the surface only for major maintenance.
To avoid the need to purchase agreements for running under surface buildings, the tunnels were bored underneath public roads, where construction could be carried out without charge. At the northern end of the railway, the need to pass deep beneath the bed of the River Thames and the medieval street pattern of the City of London constrained the arrangement of the tunnels on the approach to King William Street station. Because of the proximity of the station to the river, steeply inclined tunnels were built to the west of the station. Because of the narrow street under which they ran, they were bored one above the other rather than side by side as elsewhere. The outbound tunnel was the lower and steeper of the two. The tunnels converged immediately before the station, which was in one large tunnel and comprised a single track with a platform on each side. The other terminus at Stockwell was also constructed in a single tunnel but with tracks on each side of a central platform.
The railway was officially opened by Edward, Prince of Wales (later Edward VII) on 4 November 1890, and was opened to the public on 18 December 1890. Initially, it had stations at:
The Prince of Wales gave the following speech at the opening:
...thanks for having given me an opportunity of being present to inaugurate a work which I have but little doubt will be of the greatest use to the community, and which will especially be a great boon to this great metropolis. It must be a matter of deep thought to all of us, the ever-increasing growth of this city, and the consequent increasing difficulties of the means of access.
This, the first electric railway in England, will, I hope, do much to relieve the congestion of traffic which exists in the City. Business men who have great distances to come will by this means find an easy way of leaving the City and of enjoying the fresh air of the country. The railway will also be a material boon to the working man who is obliged to work all day in a not always pleasant atmosphere; for it will enable him also to get a little fresh air.