I don't know what they are hauling, possibly mine concentrate, or maybe just sand. Something that has to be kept dry and not allowed to blow away. Video was likely taken fairly far south in mining country.
If you want to see a gorgeous longer video of the various bridges along the line - try
That same long curving trestle - which I believe is at Channing, just outside of Flin Flon - is visible around 6:15 of the video.
PS - Google satellite seems to show an awful lot of those cars at the mine in Flin Flon.
- Paul