The Subways of North America: Transfers are tricky

Literally (if you click it):
For the pedantic rail enthusiasts, the definition of a subway used here is, with some caveats, “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.” For the rest of you, the definition is “an underground train in a city.”