v5.hvv.transport.rest is a REST API for the Hamburg public transportation system, HVV.
Because it wraps an API of HVV, it includes all local traffic, as well as some long-distance trains running in the area. Essentially, it returns whatever data the HVV app shows, including realtime delays and disruptions.
This API returns realtime data whenever its upstream, the API for DB's mobile app, provides it.
You can just use the API without authentication. There's a rate limit of 100 requests/minute (burst 150 requests/minute) set up.
This API has CORS enabled, so you can query it from any webpage.
This API sends ETag & Cache-Control headers, allowing clients cache responses properly.