Alan, copyrights and trademarks are entirely different things and subject to different legal regimes.
Expressive works - which are the subject of copyright - created by the federal government are placed in the public domain. But "Space Force" is not a work of expression; it identifies an entity and thus can serve as a mark. There is no bar to the federal government holding trademarks.
Although a mark must be "used in commerce" to receive trademark protection; we might question whether an arm of the military is really engaging in "commerce" under the "Space Force" name.
And we might further question whether any mark - whether it's "Space Force" or "Union Pacific" - is really going to cause any confusion about connection with the entity when used on a model train. If consumers do not believe there is any connection between the model train and the military branch, or the transportation company, then there would not be trademark infringement. In which case no license is necessary.
