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, Data about subway length, number of stations are available on the wikipedia website

, GMP per capita data for different cities accross the world are available on Brookings Global Metromonitor website

, Surface area and population data for urbanized areas accross the world are

, The Railisa database is available on the UICs website

, Population and surface areas of countries were taken from the Demographic Yearbook

, All the data used for this study are available in tsv