I was wondering: would it be possible to treat the "roads"
as just another type of "rail", to be used by trains of type "car"?

No need to set directions at switches/crossings,
each car would have a setting like "go left/straight/right" 10% / 70% / 20%,
(with a setting of 0% / 0% / 100% for "follow-the-right-wall" maze-running)
and move until it finds a stop (garage, busstop, gas-station, parking-space etc.),
park there and activate a random other car (or one of its own type, e.g. another bus).
Also reversing at the end of roads, and stopping at railroad-crossings and traffic-lights.

Moving the little cars on the screen would be for sure very attractive, but this is a whole project in itself!
The graphical capabilities of CATrain are extremely poor because of the technology available when it has been created. In 1992 only 16 colors were available and moving graphical "sprites" inside a window was still a performance not easy to achieve.
Adding new graphical elements is very tedious and time consuming.
The first step is to completely redesign the display routines, using true color and transparency management, in such a way that a user can easily design his own locomotives and cars.
But this is not an easy task and a first attempt failed...
Before trying again, after releasing the "Audio" version, the priority will be to extend the "Itinerary" capabilities, with "alternative" routing.

Joel