https://en.wikipedia.org/wiki/Netherlands#Functional_urban_areas
https://en.wikipedia.org/wiki/Amsterdam_metropolitan_area ,
https://en.wikipedia.org/wiki/Amsterdam#Metro,_tram_and_bus ,
https://en.wikipedia.org/wiki/Amsterdam_Metro#Rolling_stock ,
https://en.wikipedia.org/wiki/Amsterdam_Airport_Schiphol#Description
https://en.wikipedia.org/wiki/Rotterdam%E2%80%93The_Hague_metropolitan_area
