Is Transit (https://transitapp.com/) available where you live? If there's no official API for realtime updates, it also allows you to share your GPS data to the rest of the app users for crowdsourcing this info. Overall it works pretty well for me.
Oh Transit fakes data where they don't have real-time info too. They just put a slick interface on it. After waiting in the cold for a once an hour bus that they were lying about I finally uninstalled the app. The incessant prompts to pay for an Uber didn't help matters either.
Not to defend the app in question, but certain public transport organisations publish incorrect data.
One example of this is the RATP (Paris public transport) - they refuse to publish live feeds: the data they publish is a basically their "best effort" static timetable.
The data presented by the Transit app matched neither the published timetables nor reality. If memory serves it was marked as real-time data. My assumption is that Transit was using their bullshit "crowd sourced" algorithm.
Plenty of of agencies publish bogus data (e.g.: Muni has predictions for outbound train service even when a train will be taken out of service at the end of the line, as a result only the second to last prediction for the night is valid). This was Transit being useless.
