To answer your concerns:
what happens when a town starts to build on your ghost track?
Well, the beauty of design mode is that you're free to make neccessary changes. But, I guess if you're building a station, it'd be difficult to redesign the entrance. My only solution to this is maybe players can 'submit' designs to towns prior to construction approval? Like, a 'saved' design which becomes visible to other players and prevents them and towns from building on it? I say, it depends on the coder, and whether he/she is willing to put the time into it.
But remember, there's no need to build an entire line at one, in a single build mode instance.
You say potential passengers can't be seen by the player.. why not?
They can...and they can't. As I stated earlier, the maximum potential passengers
which travel from each town-pair can be seen by players, as it represents general knowledge of how many people travel from place to place. However, the specific potentials used in the Trip Generation stage represents individual passengers who may or may not use the system. Like in real life, you can only see those who choose to use your service (ie, the actual passengers).
Modelling Taxis, cars etc down there would also be an interesting challenge
Yes. Carreers have been built on modelling automobile transport patterns. Call me back in three years, and I can charge you $60,000 (at least!) to do it