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, and the other between Elephant and Castle and Stockwell, where the gradient was less, at 12 mph. 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. So, 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 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 below roadways, 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 required 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.
When opened in 1890, the line had six stations and ran for in a pair of tunnels between the City of London and Stockwell, passing under the River Thames:
The original service was operated by trains composed of an engine and three carriages. Thirty-two passengers could be accommodated in each carriage, which had longitudinal bench seating and sliding doors at the ends, leading onto a platform for boarding and alighting. It was reasoned that there was nothing to look at in the tunnels, so the only windows were in a narrow band high up in the carriage sides. Gate-men rode on the carriage platforms to operate the lattice gates and announce the station names to the passengers. Because of their claustrophobic interiors, the carriages soon became known as "padded cells". These trains were however, preserved in London Transport Museum as the first static exhibit to the metro train.
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 from Camden Town in north London to Morden in Surrey.
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 & Hampstead Railway, forming a single London Underground line called the Morden-Edgware line. In 1933, the C&SLR and the rest of the Underground Group was taken into public ownership. 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.
The CCE&HR (commonly known as the "Hampstead Tube") was opened in 1907 and ran from Charing Cross (known for many years as Strand) via Euston and Camden Town (where there was a junction) to Golders Green and Highgate (now known as Archway). It was extended south by one stop to Embankment in 1914 to form an interchange with the Bakerloo and District lines. In 1913 the Underground Electric Railways Company of London (UERL), owner of the CCE&HR, took over the C&SLR, although they remained separate companies.
During the early 1920s, a series of works was carried out to connect the C&SLR and CCE&HR tunnels to enable an integrated service to be operated. The first of these new tunnels, between the C&SLR's Euston station and the CCE&HR's station at Camden Town, had originally been planned in 1912 but had been delayed by World War I. The second connection linked the CCE&HR's Embankment and C&SLR's Kennington stations and provided a new intermediate station at Waterloo to connect to the main line station there and the Bakerloo line. The smaller-diameter tunnels of the C&SLR were expanded to match the standard diameter of the CCE&HR and the other deep tube lines.
In conjunction with the works to integrate the two lines, two major extensions were undertaken: northwards to Edgware in Middlesex (now in the London Borough of Barnet) and southwards to Morden in Surrey (then in the Merton and Morden Urban District, but now in the London Borough of Merton).
The Edgware extension used plans dating back to 1901 for the Edgware and Hampstead Railway (E&HR) which the UERL had taken over in 1912. It extended the CCE&HR line from its terminus at Golders Green to Edgware in two stages: to Hendon Central in 1923 and to Edgware in 1924. The line crossed open countryside and ran on the surface, apart from a short tunnel north of Hendon Central. Five new stations were built to pavilion-style designs by Stanley Heaps, head of the Underground's Architects Office, stimulating the rapid northward expansion of suburban developments in the following years.