I don't know if VBB used this, but most transit agencies publish their data in two standard formats these days GTFS for static schedules and GTFS-real time for real-time data. Any application you build around these formats would immediately scale to pretty much every big city.

Google maps and Apple maps provide transit directions in their apps using GTFS and GTFS real time data (partly the reason why Apple maps was able to add transit directions feature so easily - Google had to deal with the transit agencies years before that and convinced them to publish data in open source standard formats).