Even though it is a great project/product, note that it does not have realtime data in Berlin/Brandenburg. It interpolates the position based on schedule data [1].
[1] I assume based on https://daten.berlin.de/datensaetze/vbb-fahrplandaten-gtfs
[1] I assume based on https://daten.berlin.de/datensaetze/vbb-fahrplandaten-gtfs
shameless plug: I build a tool that polls VBB's (or any other compatible) API and produces a GTFS-Realtime feed. [1]