Hi Matt,
If you happen to know the Transit provider, you can have them contact us and provide their data feed as per https://www.bing.com/maps/transitcontentproviders. Otherwise the best option is when searching for the transit route on Bing.com/maps, hit the feedback button and provide the details of the expected route and start and end points. If you have it, a URL to the transit provider would also help then press "Submit". This goes directly into the feedback database for maps to be handled directly by the Maps team.
Sincerely,
IoTGirl