I haven't tried a train yet, but after making this post I did discover that I can enter and drive an "enemy" car, but I can't open its inventory. I'm guessing trains will work the same way.

Well, I wanted to maintain separate science trees for each force, but have like a public train line anyone could use or something. Is something like that possible?

So I may have missed this somewhere; but I can't seem to find it. How do you make Forces "allied" with each other so we can share chests, cars, trains, etc. Anytime I try interacting someone else's stuff, it just says "can't open enemy inventory" even though we both have our stances set at Friends.