fakeisthenewreal has a wonderful page of scaled line drawings of public subway systems from cities around the world. Surprising: the size and sprawl and the felt character of the respective cities via what look to be simple scribbles.
Transportation has such an impact on how you experience a city. It would be interesting to see how you could reflect further characteristics… ie. the NewYork system of local and express trains vs. the one-track-leads-to-all-destinations systems you often encounter in Germany. Payment systems are also profound… NYC is a gateway – once inside your inside. Berlin has an identification system… you can be controlled at any time.