I've got a similar suggestion from another person and considered it when developing the map, but I really didn't want the map to get bogged down in the details of which lines can run which cars. To me, the layout of the city and the limited line count is difficult enough to make the map a challenge.
If there's a lot of demand, though, I could add a second version of the map with line restrictions in the /extras/ folder once I've published the final maps.