The network is considered the oldest rapid transit system, it incorporates the world's first underground railway, the Metropolitan Railway, which opened in 1863 and is now part of the Circle, Hammersmith & City and Metropolitan lines; and the first line to operate underground electric traction trains, the City & South London Railway in 1890, now part of the Northern line. The network has expanded to 11 lines, and in 2012/13 carried 1.23 billion passengers, making it the 12th busiest transit system.
The system's first tunnels were built just below the surface using the cut and cover method, and are large enough to take trains of normal size. Later, smaller circular tunnels – which give rise to its nickname the --b- – were dug through the London Clay at a deeper level. The early lines were marketed as the UNDERGROUND in the early 20th century on maps and signs at central London stations. The private companies that owned and ran the railways were merged in 1933 to form the London Passenger Transport Board. The current operator, London Underground Limited (LUL), is a wholly owned subsidiary of Transport for London (TfL), the statutory corporation responsible for most elements of the transport network in Greater London.
The term '-u--' is nowadays often used both in official publicity and in general usage to embrace the whole Underground system, though the "T---" is sometimes only applied to the lines that run in deep-level tunnels, excluding the Circle, Metropolitan, District and Hammersmith and City lines.
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