Transport for Washington, US
Washington Metro transport API
Community maintained, developer-friendly public transport API.
transport.rest 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 transport.rest.

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Washington Metro transport API
The transport.rest API is provided by transport.rest transit APIs 🚂🔗 (transport.rest).
Yes. The transport.rest 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.rest API requires no authentication, so you can start making requests without signing up.
Yes. The transport.rest API supports CORS, so front-end JavaScript can call it directly.
Yes. The transport.rest API is served over HTTPS, so it's safe to use in production.