Actually, I'm not sure that would be the most northerly rail line, on several counts. First, the iron ore line to Narvik, Norway reaches 70 degrees north latitude, which looks to be about as far north as the line on Baffin Island. Second, there is a line north of Murmansk, to the White Sea, which is even a bit farther north than that. Finally, there is a short mining line near Kirkenes, Norway, currently out of service but being re-opened, that is farther north than either of those and almost certainly farther north than the Canadian line.
I've been to Narvik by rail (on an iron ore train, no less), and it's pretty far north (although, thanks to the Gulf Stream, it's forested and quite pretty). Haven't been on the other railroads.
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