b3co

wrote an interesting tool which I refound thanks to ion, where you can select logos for all subway systems you've ridden.

World metros I have taken:

Got at b3co.com!

So, Helsinki (Finland), Madrid (Spain), Porto Alegre (Brazil), Mexico City (Mexico), Caracas (Venezuela), Guadalajara (Mexico) and Valencia (Spain).

I guess people living in countries where it's easier to travel between each other, may have caught more metros