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> The universal symbol for a city’s Metro system is a big “M.”

Except in Germany, where it's a big “U” (for U-Bahn).

Or in Norway and Sweden where it is a "T"

Or any other city where they don't call it a metro

Exactly. In Argentina it's a "Subte", in Japan it's "Chikatetsu".

However, Metro seems to be the most popular name (it's used in Madrid and Sao Paulo too).

Except in Tokyo it is called the Tokyo Metro. The logo is even in the article.

I was misled by (the spanish language) Wikipedia :) , you're right, it talks about the heart shape and everything.

I can personally confirm that in Buenos Aires it's called Subte and the word Metro is used nowhere in Argentina or Uruguay.

Tokyo has both a metro and a subway (chikatetsu literally means "underground railway"). Nagoya has a subway, and the logo is a kind of tunnel.