With 420km of network, Shanghai's metro overtook the London Underground, which has a total of 402km. But the rate of expansion is more impressive: the first line was constructed in only 1995 and it is still expanding.
By 2020 Shanghai, "intends to have added over 350km in new lines and extensions, almost doubling its network length."
However, "there is no suburban commuter rail system in Shanghai that compares with those in cities like London, Paris and Tokyo, where the railway network is essentially operated as a secondary rapid transit system with longer station intervals than the subway, generally with an interchangeable fare system."
Chinese cities have 8-10 times the density of our densest cities, and so metros make lots of economic sense there.