The webcomic xkcd put all real the subway systems of North America on one map, joined by imagined lines connecting the systems far away from another. Helpfully the author defines for us what he means by “subway”, for “the pedantic rail enthusiasts”:
a network containing high capacity grade-separated passenger rail transit lines which run frequently, serve an urban core, and are underground or elevated for at least part of their downtown route.
Sounds good enough to me.
Did you know that cities like Baltimore, Santo Domingo and San Juan have subway systems? That one in three subway stops are in NYC? That Springfield is located somewhere between Vancouver, Toronto and San Francisco? Well, now you know.