It is the front piece of a Trainman's work-overalls/coveralls rivet-button.
It's called a rivet-button because it didn't have a loop on the back, but instead is the wide disc on a rivet, which the overalls' straps "buttoned" onto. Some bluejeans still come with a rivet-button atop the zipper, or a set of them in place of a zipper. (I'm wearing some jeans like that now.)
The rivet-button front you found dates from approximately the late-1800s into the first half of the 20th-Century. There are many-many variations showing a train-engine. I haven't saved any photos of them because I'm not "into" that kind. But perhaps some other folks here have such photos.