General Transit Feed Specification
The General Transit Feed Specification (GTFS), originally called the Google Transit Feed Specification, was developed by Google as a format for sharing public transportation information such as bus stops and bus routes.
Looking on Taginfo, some of the tags that are currently in use include: gtfs_id, gtfs_stop_code, gtfs_route_id. However it doesn't appear that anyone has stored the agency_id. There's also the possibility that a single stop could have multiple carriers (operators), and therefore should be able to have multiple pairs of agency_id and stop_id.
- GTFS Data Exchange - Data available for 1000 transit agencies (as of 9 Dec 2016), though licensing varies. Soon to be shutting down.
- Google's list of GTFS data sources
- TransitFeeds - open source aggregation project of GTFS data.
- transit.land - commercially funded aggregation of GTFS data.
- GO-Sync - a GTFS and OpenStreetMap data synchronization tool - a Google Groups thread announcing gtfs-osm-sync, and difficulties of multiple operators for bus stops
- GO-Sync - a GTFS and OpenStreetMap data synchronization tool - gtfs-osm-sync announcement on Talk-transit
- GTFS compatibility (and  and ) - discussion on Talk-transit
- Bus stops in North America from GTFS data - thread on Talk-transit
- GO-Sync (aka gtfs-osm-sync) - a desktop tool to synchronize GTFS feeds with OSM