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London underground revealed

The metro system in London is the biggest one in the western world (second to the one in New York city). This system moves millions of passengers everyday and it is an inevitable component of London’s daily operation. This metro is successful based on other ingenious inventions applied to famous metro systems. Four of them are mentioned in the movie: 
1. City & south London Railway: 6 km
2. New York city subway: 15 km
3. Paris metro: 91 km
4. London underground: 402 km
Four forward leaps which allow the metro system bigger and bigger: 
Leap 1: Tunnels
Leap 2: Propulsion
Leap 3: Station
Leap 4: Safety 
Episode 4
Session 3
The copyright belongs to National Geographic Channel.
ThuVienXayDung.net makes the subtitles.