Delays in subway lines.
Metro Lisboa API is a free Transportation API that requires no API key, is only available over HTTP and does not support CORS, so it should be called from a server.

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Yes. The Metro Lisboa 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 Metro Lisboa API requires no authentication, so you can start making requests without signing up.
Not directly. The Metro Lisboa API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
No. The Metro Lisboa API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.