One of my favorite maps that I've covered here has been this map from radical cartography showing the mass transit systems of North America at the same scale. Recently, via Matt Yglesias, I came across a similar project: this one is by Neil Freeman and it shows subway systems from around the world at the same scale. Here are some of the subway systems he's got, though there are many others.
I - an American who detests the standard mode of American urban development of the last 60 years - tend to think of European cities as walkable utopias with comprehensive mass transit systems. I don't know if that's quite a fair judgment, but one thing you notice from these maps is that, other than New York City, there are very few American cities with a subway system that forms a dense web like Paris' or Madrid's. Even Washington, which has a very good subway system, is sort of spread out, and then it goes downhill from there. But a bunch of cities in Europe and Asia have very dense systems, and it seems that that's what you need to have truly comprehensive rail mass transit: the ability to walk to the nearest Metro stop, and then to get from pretty much any point in a city to pretty much any other point on the subway. You can do that in New York City; I don't know if you can do it in any other city in North America.