I think something that has failed to be mention here are the following considerations. First, it would be costly to keep those rails de-iced. Second, this line would make arctic oil drilling much more attractive (with all the consequences that carries with it, both positive and negative). Third, increasing trade between any two countries improves their political relations. Fourth, Canada is not a country, it's the 51st state (j/k).
Overall, I am inclined to agree with stratagos, there are much better ways to spend that kind of money. However, IMHO, best options seem to always get thrown off the rails by small-minded interests or poorly designed legal frameworks. This rail line would be cool, it would generate revenue, but it wouldn't be enough revenue to make it worthwhile. Even if carbon costs are included, I think the difference between sea and rail is not substantial enough to justify the investment. Also, the project would likely fail from lack of funding because it would certainly be seen as "wasteful spending." Please remember that the US has rather let cities flood and bridges collapse than put money into infrastructure. As much as it pains me to say it, the days of grand infrastructural projects, no matter how beneficial, is over. All such spending has been deemed "wasteful" in the minds of too many American voters.