While not a solution right now, I just want to add that the general transit feed spec aims to solve the data interoperability of different transit systems. The transit system keeps a publicly accessible zip file up to date, and then anyone can pull/parse the schedule/prediction data in a consistent way across transit systems. I know in the us adoption is slow, with vendors prefering to build their own walled gardens and transit agencies lacking the vocabulary or skills to advocate for more open data/tools

I used Öffi before too, but Transportr supports more countries and looks nicer imo :)