CTS Realtime API.
CTS API is a free Transportation API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Cruise sailings, day-by-day itineraries, weekly fare history, port traffic and a cruise price index
Yes. The CTS 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 CTS API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The CTS API supports CORS, so front-end JavaScript can call it directly.
Yes. The CTS API is served over HTTPS, so it's safe to use in production.