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, 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 – and extended the line by a further . Before the railway opened, a further bill received assent, granting permission to continue the line south to Clapham Common. The act was published on 25 July 1890 as the City and South London Railway Act, 1890, also effecting a change of the company's name. Haulage and infrastructure Because of the small diameter of the tunnels as well as the difficulty of providing sufficient ventilation, the use of steam power, as used on London's other underground railways, was not possible 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 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 , and the other between Elephant and Castle and Stockwell, where the gradient was less, at . 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 (This is now a fourth rail) beneath the train,