Transport for Belgium
The iRail API is a third-party API for Belgian public transport by train

Third-party VBB API.
Transport for Berlin, Germany API is a free Transportation API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
An HTTP API for Berlin & Brandenburg public transport.
Details from github.com.

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The iRail API is a third-party API for Belgian public transport by train
The Transport for Berlin, Germany API is provided by derhuerst (github.com). An HTTP API for Berlin & Brandenburg public transport.
Yes. The Transport for Berlin, Germany API is listed in the Transportation category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Transport for Berlin, Germany API requires no authentication, so you can start making requests without signing up.
CORS support for the Transport for Berlin, Germany API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Transport for Berlin, Germany API is served over HTTPS, so it's safe to use in production.