Are there rail lines that get closed over the winter months because of snow conditions, or have modern technology and equipment overcome that?
Hudsons Bay Railway still pulls the plug. (simple economics and sometimes polar bears)
On the Donner Pass from Truckee to Colfax if the snow is to heavy for the engines snow plow to clear the tracks, first they bring out the Jordon Spreader. If it a blizzard and the snow piles up feet deep which is not uncommon in the winter, they call out the rotary plow which is very expensive, BUT they have to keep the line open. One rotarty is based in Sparks, NV and the other in Roseville, CA.
Not a common carrier, but the agreement between the Adirondack RR and NYS is that from December 1 to April the corridor is a snowmobile trail.
There have been occasions in the fall when that December 1 date was slipped a bit, but usually there was no snow of significance on the ground yet.
My understanding is that VIA runs to Churchill year-round.
The roadbed should also be able to handle heavier freight loads in winter (frozen ground), though the port would of course be closed.
The polar bear hazard might actually be higher in summer, as the lack of ice forces them to stay on land.
I don’t know about current lines, but the UP used to shut down the West Yellowstone branch (now abandoned) in the winter.