done successfully. However, reservations over the stability of underground tunneling persisted into the 1860s and were finally overcome when Parliament approved the construction of London's first underground railway, the Metropolitan Railway. Begun in 1860 and completed in 1863, the Metropolitan inaugurated the world's oldest mass transit system, the London Underground; it was created by the cut-and-cover method of excavating a trench from above, then building reinforced brick walls and vaults to form the tunnel, and filling in the trench with earth. The Metropolitan initially ran from Farringdon in the east to Paddington in the west. The open wooden passenger cars were propelled by a coke-fueled steam locomotive and lit with gas lamps to provide illumination in the tunnels. The line was such a success, carrying 9.5 million passengers in its first year of operation. An extension to the western suburb of Hammersmith was built and opened in 1868. In 1884 the Metropolitan was linked with the Metropolitan District Line at Aldgate to form an inner circle encompassing Central London (the modern Circle Line), with a short stretch running from Aldgate to Whitechapel. In 1880 an extension line to the north-west was opened from Baker Street tube station to the village of Harrow, via Swiss Cottage and St. John's Wood, expanding further over the coming decades and allowing prosperous suburbs to be developed around formerly rural villages. A succession of private enterprises were formed to build new routes after the success of the Metropolitan, the first being the Metropolitan District Line, opened in 1865. This line extended along the Thames from Westminster to South Kensington at first, but by 1889 it had been extended eastwards to Blackfriars, and south-west all the way to Wimbledon. The next line to be built, and the first true "tube", dug with a tunnelling shield designed by J.H. Greathead rather than by cut-and-cover excavation, was the City and South London Railway, opened in 1890. It was London's first underground line with electric locomotives, and the first to extend south of the river. Electrification allowed the tunnels to be dug deep below ground level, as ventilation for smoke and steam was no longer necessary. In the year 1894, an estimated 228,605,000 passengers used the three underground railways then in operation, compared to 11,720,000 passengers in 1864 using the lone Metropolitan Railway. Before the century came to a close, a second deep-level tube was opened in 1898: the