Thanx for responding! I have never added any modified buildings. I play the game every once in a while, never long enough for an extensive search for extras. What I should have mentioned btw, is that I play Simcity 4, without the Rush Hour expansion. What I have done is wrongly installed the NAM Traffic Subsystem. I found out it wouldn't work after installing it... But my other (smaller) cities do not have this public transport lock-down problem.
It might be a lack of RAM memory (just the 1 GB on my laptop). But I've built bigger cities without problems before.
Oh, and when I build a new station/substation/bus stop, it does work. I could replace all the existing stations, but with a city this big, I might go bankrupt.. So any other suggestion would be very welcome.