Wow, what a quick response. Good to hear OSM has useful stuff.
For me, all I want is a Map. Just like the standard subway/metro/tube maps that get handed out in every city. Except the routes and stops would be superimposed on the PE map so that I could see what stops/stations/etc. were nearby, and what lines connected to what.
The route lines on the map would be simple connect-the-dots between stations. They would be colored to match the official city-produced diagram/map. The connections would be shown with the fairly standard circles.
Interactive highlighting would be cool, but I’d prefer to see a map layer overlay for the entire system first. With such, it would be easy enough to work out one’s own route just as we do with a printed or PDF map. Interactive highlighting of a single route doesn’t sound, on the face of it, very useful. It’s often, perhaps usually, necessary to make transfers to get from Here to There, and I’d like to see the possibilities all at once!