IIRC, from discussions that I have followed over the past several years, it is indeed to allow the Canadians to export Tar Sands oil to Asia. However, there is no place left to build a commercial sea port on Canada's west coast.
What they propose to do is to build a standard gauge railroad from the Fort McMurray, AB area to Delta Junction, AK in order to feed that oil into the Trans Alaska Pipeline. The Pipeline is seeing a steady long-term decline in traffic volume due to the North Slope oil fields being played out, so there is capacity in it to carry the Tar Sands oil from Delta Junction to the sea port at Valdez, AK.
The expected volume of traffic that I have seen discussed could very well require that that rail line be built as double track, so it would certainly be doable from an economic standpoint and Delta Junction is close enough to the end of ARR's (Alaska Railroad) track at nearby Eielson Air Force Base, just southeast of North Pole, AK, that connecting them would not be a major stretch.
It is not proposed to go anywhere near the never completed BCOL line at Dease Lake, BC.
I suppose that once things are up and running, it could also be attractive to passenger operators to offer multi day excursions through some of the most wild and unspoiled scenery on the planet, much like Amtrak does on some of their current long distance runs (ie, the ever popular Empire Builder and California Zephyr), but it would primarily be a freight route.
Mike