v6.bvg.transport.rest is a REST API for the Berlin public transportation system, BVG.
Because it wraps a BVG API of BVG, it includes all local traffic of Berlin & Brandenburg, as well as some long-distance trains running in the area. Essentially, it returns whatever data the BVG app shows, including realtime delays and disruptions.
This API returns realtime data whenever its upstream, the API for BVG'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
Cache-Control headers, allowing clients cache responses properly.