The iRail API is a third-party API for Belgian public transport by train.
Transport for Belgium API is a free Transportation API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from docs.irail.be.

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The Transport for Belgium API is provided by iRail API Docs (docs.irail.be).
Yes. The Transport for Belgium 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 Belgium API requires no authentication, so you can start making requests without signing up.
Yes. The Transport for Belgium API supports CORS, so front-end JavaScript can call it directly.
Yes. The Transport for Belgium API is served over HTTPS, so it's safe to use in production.