These are the Gunderson AutoMax vehicle transport cars. They are a tripple-decker rail car that is used to transport cars and trucks via rail. The chains they weld to the bottom are to hold the cars in place as the train moves, with racheting chain hooks much like flat bed tow trucks use. They have been built here in Portland for decades.
Every picture I have seen with the caption of "FEMA RAILCARS!" has been a normal, Gunderson car, no FEMA or UN markings. They usually have a railroad company insignia. In the pictures in the video you posted, at the 3:50 mark, you can actually see a truck on the top deck of one of the cars if you look closely. And I had to laugh a little when the video says "LOOK, MORE RAILCARS!" and shows regular flat-deck railcars lined up out in the rail yard.
I've seen many of these, and they have nothing to do with FEMA.
Some of the stuff in that video was just grasping at straws. The mail van with plain-clothes people in it? Mail truck drivers in my area aren't usually wearing a uniform. Sometimes they do?
(Sigh) there's this kind of stuff, then there's the kind of stuff that should really scare us. Boxingnun, I don't mean to be a jerk, but the FEMA railcar thing has been re-hashed so many times...