I have to say all the input from our technical experts on this forum is impressive - thank you all.
Here's a CBC News article referencing a CP white paper on winter train operation. The thing that jumped out at me was the recommended reduction in speed during frigid temps. If the reduction amount is close to a downgrade speed limit, the suggestion seems to be no running trains below -35C (-31 F) in that area. I assume that wouldn't happen.
"The white paper also said train speeds must be reduced in frigid temperatures — by at least 16 km/h below –25 C and by at least 32 km/h at –35 C." https://www.cbc.ca/news/canada...ment-fatal-1.5011096