VIA Rail, Canada's version of Amtrak, has passenger service between Winnipeg and Churchill, Manitoba, over the Hudson's Bay Railway (formerly owned and operated by Canadian National RR).  Check with   (You need to Login or Register .

Other than that, this book claims to cover a lot of Canada north of the populated area.  Ontario Northland? That RR serves from north of Toronto to Temiskaming, I think.  So a pretty wide area.