VBB has been publishing GTFS (Static/Schedule) data for almost 10 years now. [1][2]
But there is no (truly open) realtime data (e.g. GTFS Realtime or SIRI) available because their API [3]
- requires signing a draconian contract (e.g. ridiculous liability clauses, no permission to pass the data on in any form), and
- API works individual vehicles/trips, so you'd have to poll every single one out there to get the equivalent of a GTFS-RT dataset.
There is an unofficial API though [4][5] that is de-facto open, and I have built a tool that pools the data and creates a GTFS-RT feed. [6]