Yes, I know it takes some time for the passengers to recognize the route but I ran it for a couple of months and they there was just no one who wanted to travel from one town to the other.
You asked for a savegame but since I didn't save this attempt I quickly recreated it as exactly as I could and for some reason it works now.
I really don't know what I did wrong before, I carefully doublechecked all the connections. All the busses and the train where driving their route. Strange.
Anyways, thanks for your help, somehow it solved my problem and answered my question.