Any number could be coupled together and all the motors controlled by the driver at the front. In 1898 the deep-level Waterloo and City Railway opened with EMUs, a central third rail providing traction current at 500 V to 530 V DC. The current for the two motor cars could be controlled from either driving cab, but the Board of Trade became concerned about the fire risk presented by the 11 power cables carried on the roof of the train and barred installation of similar systems in deep-level tubes. Therefore, when in 1900 the Central London Railway opened its deep-level tube railway, electric locomotives hauled carriages. In 1900 the Metropolitan Railway and the District Railway ran an experimental EMU in passenger service for six months. A four-rail system was used, two rails outside the running rails providing the traction current at 500–550 V DC. Only the traction motors in the leading motor car were used, the trailing motor coasting. In 1901 a Metropolitan and District joint electrification committee recommended a three-phase AC system with overhead wires. Initially this was accepted by both parties, but the District needed to raise finance and found an investor in 1901, the American Charles Yerkes. Yerkes soon had control of the District and his experiences in the United States led him to favour DC with a track level conductor rail pickup similar to that in use on the City & South London Railway and Central London Railway. After arbitration by the Board of Trade the DC system was taken up. In 1897 Frank J. Sprague demonstrated a system where a low-voltage control signal from any driving position controlled all the motors on a train, developed for the South Side Elevated Railroad in Chicago. This electro-magnetic system was developed in Britain by British Thomson-Houston (BTH) and first used in Europe in 1902 on the Central London Railway when it changed over to EMU operation to solve a design problem with its locomotives. Westinghouse Electrical developed an electro-pneumatic multiple unit system that used a lower control current and when in 1903 the District opened its line from Acton Town to South Harrow with experimental EMUs these used control systems from BTH and Westinghouse. After these trials the District selected BTH equipment for its B Stock. The underground Mersey Railway started an electric service on 3 May 1903, electrified by British Westinghouse Electric with equipment from the US. A four-rail DC system was installed, with a positive outer rail and a negative return rail between the running rails. In London the Great Northern & City Railway was a underground line from to .