
Chicago Transit Authority (CTA).
Transport for Chicago, US API is a free Transportation API that requires apiKey authentication, is only available over HTTP and has unconfirmed CORS support.
The CTA's Developer Center offers a variety of data feeds that allow anyone to develop Web, mobile and other applications to help transit riders throughout the Chicago area.
Details from transitchicago.com.

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Deutsche Bahn (DB) API
The Transport for Chicago, US API is provided by CTA (transitchicago.com). The CTA's Developer Center offers a variety of data feeds that allow anyone to develop Web, mobile and other applications to help transit riders throughout the Chicago area.
Yes. The Transport for Chicago, US 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 Transport for Chicago, US API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Transport for Chicago, US API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
No. The Transport for Chicago, US API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.