That would explain why I‘ve never heard of that trestle (because it’s on the freight-only branch to Flin Flon, not the actual Hudson Bay Railway) and that that freight train crawls so slow (because the tracks aren’t maintained for freight trains)…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.