smiling Fuzhou Metro train
#TrainTwitterpic.twitter.com/Gv4JEA4JKN
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underground vs aboveground distinction is not very important; the busiest rail lines in Tokyo are all above ground (but they're not trams either!)
more to the point though, you want to build a system that reflects the number of people who are trying to use it--larger cities need more routes with higher capacity, etc
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