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Construction of London’s subway began in the 1860’s. Over the next 70 years, as demands for public transportation grew, the system developed haphazardly until by the 1930’s it had reached much the form it has today: a network as sprawling as the city it serves.
Original Article Source: http://trifter.com/europe/united-kingdom/the-london-underground-an-amazing-subway-simplification-and-mathematical-solution-for-travelers/