As I was walking home yesterday, I heard a man say to someone, on his cellphone, that he was on his way to the "L" and my first thought was, "He's not from around here." But then it got me thinking... what you call the subway says alot about where you're from. I know that this guy is from Chicago and he is either travelling on business or has just moved here. When I lived in Toronto, I took the subway. Then I moved to Boston and it was the "T." Now that I live in the DC area, I call it the Metro. Perhaps one day we'll move to London and I can ride the "Tube!"
The Wikipedia article on rapid transit speaks to this, "There is no single term in English that all speakers would use for all rapid transit or metro systems." In fact, there's a whole other article that lists all the rapid transit systems around the world.
Here's a picture of Max in Boston from last year. Tell me, what do you call the public transportation in your city? Extra points if you've lived in more than one place with a subway or have travelled and know the local term for it.