Washington Metro transport API.
Transport for Washington, US API is a free Transportation API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
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Details from developer.wmata.com.

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Community maintained, developer-friendly public transport API
The Transport for Washington, US API is provided by Microsoft Azure API Management - developer portal (developer.wmata.com). Discover APIs, learn how to use them, try them out interactively, and sign up to acquire keys.
Yes. The Transport for Washington, 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 Washington, US API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the Transport for Washington, US 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 Washington, US API is served over HTTPS, so it's safe to use in production.