Monument station) and Elephant and Castle. From Elephant and Castle, the tunnels were a slightly larger to Stockwell. This was a legacy of the original intention to haul the trains by cable. The tunnels were bored under the roads to avoid the need for agreement with owners of property on the surface. The original intention to cable-haul the trains changed to electric power when the cable company went bankrupt. A conductor rail energised with +500 volts DC conductor rail for the northbound tunnel and −500 volts for the southbound laid between the running rails, though offset from the centreline, powered the electric locomotives that hauled the carriages. The carriages were fitted with small windows and consequently were nicknamed padded cells. By 1907, the C&SLR had extended from both ends, south to Clapham Common and north to Euston. In 1898, the Waterloo & City Railway was opened between London & South Western Railway's terminus at Waterloo station and a station in the City. Operated by the L&SWR, the short electrified line used four-car electric multiple units. Two diameter tunnels were dug beneath the roads between Shepherd's Bush and Bank for the Central London Railway (CLR). In 1900 this opened, charging a flat fare of 2d (approximately p today), becoming known as the "Twopenny tube" and by the end of the year carrying nearly 15 million passengers. Initially electric locomotives hauled carriages, but the heavy locomotives caused vibrations that could be felt on the surface. In 1902–03 the carriages were reformed into multiple units using a control system developed by Frank Sprague in Chicago. The CLR was extended to Wood Lane (near White City) in 1908 and Liverpool Street in 1912. The Great Northern & City Railway was built to take main line trains from the Great Northern Railway (GNR) at Finsbury Park to the City at a terminus at Moorgate. However the GNR refused permission for trains to use its Finsbury Park station, so platforms were built beneath the station instead and public service on the line, using electric multiple units, began in 1904. District and Metropolitan electrification On the District and Metropolitan Railways, the use of steam locomotives led to smoke-filled stations and carriages that were unpopular with passengers and electrification was seen as the way forward. Electric traction was still in its infancy and agreement would be needed between the two companies because of the shared ownership of the