Live bus and rail departures, vehicles, disruptions and journey planning for Great Britain.
TransCAPI is a free Transportation API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
UK public transport API: bus and rail timetables, live buses and trains, disruptions and journey planning across Great Britain and Ireland, with one key.
Details from transcapi.com.

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The TransCAPI is provided by TransCAPI (transcapi.com). UK public transport API: bus and rail timetables, live buses and trains, disruptions and journey planning across Great Britain and Ireland, with one key.
Yes. The TransCAPI 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 TransCAPI uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The TransCAPI supports CORS, so front-end JavaScript can call it directly.
Yes. The TransCAPI is served over HTTPS, so it's safe to use in production.