Amazing film. I love how some of the dougboys have some French gear as they're building track.
As for the odd caps, I'm also unclear who they are. Those aren't anything I know of that were issued to the US Army in France by anyone. They could be some kind of French colonial troops helping with manual labor? There are clearly some French soldiers working on the track in another part of the film.
Note the campaign hats on several of them, as those weren't worn for long in France.
-Lee
Flickr photo set of my On30 layout