So it's possible I would get them at the same rate as now, even if I would use them all up? The trains like to bring stuff we don't need, so i don't know how that's possible

If i remember correclty I used to get at least one crate/3 trains, but usually 2/3, now I am getting like 1 crate/6-9 trains. Might be a coincidence, I guess I test it right now, but there is something I noticed with the materials. Even if I am not building a community building and I sell all the building materials right away, I am barely getting any barn tools. As soon as I start to stockpile some building materials, I start getting twice as much barn tools. That can't be a coincidence. It should affect your trains if you run out of something, at least a little bit.