I asked UPS support and got the answer:
The < freightRate:TimeInTransit > will only be returned when posting a request to the production URL.
Your key will need to be approved for production access for the Ground Freight Rating API before you will be able to post successfully to the production URL.
The .xsd files and WSDL in the developer tool kit are designed to deliver this element in production without editing, so what you have observed with the schema files is by design.
And this is example response with TimeInTransit tag from UPS support https://gist.github.com/antongorodezkiy/5973888