Tanks a lot for the video. I noticed a single covered hopper directly behind the trailing locomotive; is that there because RR's are not allowed to have tank-cars containing flammables/explosives/volatile products/ directly connected with a locomotive? If that is the case then I'm assuming that the single covered hopper tagged onto the end is there for the times the train would need a pusher.
Those are indeed dedicated Buffer Cars. Many railroads have a pool of such cars just for that purpose, and generally label them as such.
I assume those dedicated cars are weighted in some manner, as having an empty and thus light weight car, being used in such a manner doesn't seem to me to be a good thing.