Stations and predicted arrivals for BART.
Bay Area Rapid Transit API is a free Transportation API that requires apiKey authentication, is only available over HTTP and has unconfirmed CORS support.
Details from api.bart.gov.

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The Bay Area Rapid Transit API is provided by BART (api.bart.gov).
Yes. The Bay Area Rapid Transit 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.
Yes. The Bay Area Rapid Transit API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Bay Area Rapid Transit API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
No. The Bay Area Rapid Transit API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.