Couple maps include "metros" NYC and Vancouver. Couple use the word "metro" (Cairns and Sydney) but don't actually use subway systems. Then the Rail series of maps of course use trains but they are overland, continental systems.
It'd be a good idea to use just a subway system, and natty has already mentioned the London subway (
not the Underground, copyrighted...) system map in the draft room. Plenty of great cities have subway systems and could considerably make great maps. 2 things, 1 good, 1 bad: Most subway systems are going to have maps that you can draw inspiration from... such as this map of the SEPTA rail system that serves Greater Philadelphia:
- Click image to enlarge.

Bad thing is that like the London Underground copyright, most names of subway systems are copyrighted, so in this case you could NOT use the name for the subway system of Philadelphia, SEPTA. (SouthEastern Pennsylvania Transportation Authority)
