It's not a complicated one, but I like Good Magazine's summary of the biggest train systems in the world (top 5 U.S. cities and top 5 foreign cities).  The silhouettes represent the daily rides in the city, and the length of the train shows how many miles that system covers.  To the right is a quick map of each city's subway system and some statistics about their subway system.

Thanks Li, for sending in the link!