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 . EMUs with BTH electrical equipment were used from the opening on 14 February 1904; these picked up traction current from conductor rails both sides of the running rails. When the Lancashire and Yorkshire Railway (L&YR) was considering electrification of its Liverpool suburban railways, concerns were being expressed, especially in the railway press, that engineering developments in Britain were being overtaken by electrification projects in America and Switzerland. The Preston-based Dick, Kerr & Co. was made responsible for the traction systems and the L&YR built the rolling stock. A 625 V DC four-rail system was adopted, the live rail outside the running rails with a return rail, cross-bonded to the running rails, between them. The two driving motor cars each powered by four 150 hp (110 kW) motors directly controlled from a driving cab at either end of the train. Electric services started between and on 22 March 1904. The North Eastern Railway (NER) began using EMUs between and on 29 March 1904 and from 25 July 1904 over a circular route between and Tynemouth, electrified with a third rail at 600 V DC (see Tyneside Electrics). The EMUs, with BTH electrical equipment, were built at the NER Carriage Works at York with clerestory roofs. The Metropolitan Railway began its electric services from Uxbridge to Baker Street in 1905 with EMUs using Westinghouse electric equipment; these ran off-peak as three-car units with a motor car and a driving trailer. Twenty trains with BTH equipment were ordered for the Hammersmith & City line, jointly operated by the Great Western Railway and the Met. From June 1905 to the end of 1905 EMUs replaced steam on all District services. In 1906-07 three deep-level tube railways opened: the Charing Cross, Euston and Hampstead Railway, Great Northern, Piccadilly and Brompton Railway and Baker Street and Waterloo Railway, all subsidiary companies of the Underground Electric Railways Company of London.