The London Underground is a public transport network, composed of electrified railways (that is, a metro system) that run underground in tunnels in central London and above ground in the city's suburbs. The oldest metropolitan underground network in the world, first operating in 1863, the London Underground is usually referred to as either simply "the Underground" by Londoners, or (more familiarly) as "the Tube".
Since 2003, the Tube has been part of Transport for London (TfL), which also schedules and lets contracts for the famous red double-decker buses. Previously London Transport was the holding company for London Underground.
There are currently 275 open stations and over 253 miles (408 km) of active lines, with three million passenger journeys made each day (927 million journeys made 1999-2000; there are a number of stations and tunnels now closed).
Lines on the Underground can be classified into two types: sub-surface and deep level. The sub-surface lines were dug by the cut-and-cover method, with the tracks running about 5 metres below the surface. Trains on the sub-surface lines have the same loading gauge as British mainline trains.
The deep-level or "tube" lines, bored using a tunnelling shield, run about 20 metres below the surface (although this varies considerably), with each track running in a separate tunnel lined with cast-iron rings. These tunnels can have a diameter as small as 3.56m (11ft 8.25in) and the loading gauge is thus considerably smaller than on the sub-surface lines, though standard gauge track is used.
Lines of both types usually emerge onto the surface outside the central area, the exceptions being the Victoria Line which is in tunnel for its entire length save for a maintenance depot, and the Waterloo & City Line which, being very short, has no non-central part and no surface line.
Each station displays the Underground logo containing the station's name in place of the word "Underground", both at entrances to the station and repeatedly along the station walls so that they can easily be seen by passengers on arriving trains.
In addition, many stations' walls are decorated in tile motifs that are unique to the station, such as profiles of Sherlock Holmes' head at the Baker Street station or a cross containing a crown at the King's Cross station.
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