Because it wraps [an API](https://github.com/public-transport/hafas-client/blob/master/readme.md#background) of [Deutsche Bahn](https://de.wikipedia.org/wiki/Deutsche_Bahn), it **includes most of the long-distance and regional traffic, as well as some international trains and local buses**. Essentially, it returns whatever data the [*DB Navigator* app](https://www.bahn.de/p/view/service/mobile/db-navigator.shtml) shows, **including realtime delays and disruptions**.
This API returns realtime data whenever its upstream, the [API for DB's mobile app](https://github.com/public-transport/hafas-client/blob/33d7d30acf235c54887c6459a15fe581982c6a19/p/db/readme.md), provides it.
You can just use the API without authentication. There's a [rate limit](https://apisyouwonthate.com/blog/what-is-api-rate-limiting-all-about) of 100 requests/minute (burst 150 requests/minute) set up.