@elilla KDE Itinerary is open-source and supports real-time data (which means accounting for delays and changes etc.) but I'm not sure if it supports specifically the realtime API *for DB*. It does seem likely though, I think some of the developers are German?
@elilla (Saw your edit with requirements, and it ticks all those boxes, insofar the relevant transport providers are supported anyway because not all have available open datasets)