[custom_adv] The London Underground (also known simply as the Underground, or by its nickname the Tube) is a public rapid transit system serving London, England and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in the United Kingdom. [custom_adv] The Underground has its origins in the Metropolitan Railway, the world's first underground railway. Opened in 1863, it is now part of the Circle, Hammersmith & City and Metropolitan lines; the first line to operate underground electric traction trains, the City & South London Railway in 1890, is now part of the Northern line. [custom_adv] The network has expanded to 11 lines, and in 2016–17 carried 1.379 billion passengers, making it the world's 11th busiest metro system. The 11 lines collectively handle approximately 4.8 million passengers a day. [custom_adv] The system's first tunnels were built just below the surface, using the cut-and-cover method; later, smaller, roughly circular tunnels—which gave rise to its nickname, the Tube—were dug through at a deeper level. The system has 270 stations and 250 miles (400 km) of track. [custom_adv] Despite its name, only 45% of the system is underground in tunnels, with much of the network in the outer environs of London being on the surface. [custom_adv] In addition, the Underground does not cover most southern parts of Greater London, with less than 10% of the stations located south of the River Thames. [custom_adv] The early tube lines, originally owned by several private companies, were brought together under the "UndergrounD" brand in the early 20th century and eventually merged along with the sub-surface lines and bus services in 1933 to form London Transport under the control of the London Passenger Transport Board (LPTB). [custom_adv] The current operator, London Underground Limited (LUL), is a wholly owned subsidiary of Transport for London (TfL), the statutory corporation responsible for the transport network in Greater London. [custom_adv] The idea of an underground railway linking the City of London with the urban centre was proposed in the 1830s,[and the Metropolitan Railway was granted permission to build such a line in 1854. [custom_adv] To prepare construction, a short test tunnel was built in 1855 in Kibblesworth, a small town with geological properties similar to London. This test tunnel was used for two years in the development of the first underground train, and was later, in 1861, filled up. [custom_adv] The world's first underground railway, it opened in January 1863 between Paddington and Farringdon using gas-lit wooden carriages hauled by steam locomotives.