v6.vbb.transport.rest is a REST API for the public transportation system of Berlin & Brandenburg, VBB.
Because it wraps a VBB API, it includes all local traffic, as well as some long-distance trains running in the area. Essentially, it returns whatever data the VBB app shows, including realtime delays and disruptions.
This API returns realtime data whenever its upstream, the API for VBB's mobile app, provides it.
You can just use the API without authentication. There's a rate limit of 100 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.