Transport for Denver, US vs Transport for Norway

RTD Denver's original gtfs-developer-guide.shtml page is gone after a full site rebuild (now 404s); the transit agency's open-data offering now lives under open-records/open-spatial-information, split into static GTFS zip downloads (bus, light rail, commuter rail, plus the regional Bustang service) and three GTFS-Realtime protocol-buffer feeds (vehicle positions, trip updates, alerts). No API key is required, only acceptance of a redistribution license agreement.

Entur is Norway's national registry for public transport data. It combines schedules and real-time updates from more than 60 operators, with 21,000 departures a day. It offers a GraphQL Journey Planner at https://api.entur.io/journey-planner/v3/graphql, a REST geocoder (v3) for address and place lookups, SIRI real-time feeds, and NeTEx static feeds. Access is free and keyless for read use. Entur asks each consumer to add an ET-Client-Name header, and it may rate-limit traffic that doesn't. The old developer.entur.org portal still publishes API changes, but Entur will retire it once its content moves to developer.entur.no.