Explicație:
The London Underground is the oldest tube network in the world and at the same time a key component of the public transport system in London. It is also familiarly called the Tube. In addition to its practical function, the London Underground is also a historical monument and an independent tourist attraction. In 2000, it was used by 3 million people daily .The London Underground serves 12 lines. They can be classified into two types: shallow and deep. The surface lines are about 5 m below the ground and were built by digging and covering. The gauge of these lines is normal. The depth lines are made in circular tunnels, and are at depths of 20 to 50 m. The diameter of the tunnels is small, only 3.6 m in some cases, so the track gauge is smaller than the standard
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