Unless there's some incredible traffic change that requires many cars moving betwixt Seattle and Anchorage, any continuous rail line won't happen. (It would be nice if it did, though). The BC-funded barge lines use the Seattle-Alaska traffic, revenue and profit to cover losses elsewhere in their network (just like rail service to Churchill and Moose Factory, it's a at-loss service), and the BC and CAN gov't both know that a direct rail line will result in a much higher subsidy payout. Job protection is an isue too. (Why they don't tell them that rail work is better,I don't know)
The closest anyone has gotten to a continuous line involved the BCRY's Dease Lake Extension, the fampus unfinished line to nowhere. the barge lines bitterly fought it, and even after many promises NOT to make it a through line, tehy still managed to make BCRY shorten it. Contractors that won grading or construction bids simply didn't show up, and the line was never completed to even the barge line's "demanded" short plan. Plus, a chunk of the northernmost line was abandoned some time later.
there was an article in TRAINS about this some years ago; the convoluted bov't happenings almost required a scorecard to keep track of things.
If you checked out the neat link to the barge pic (above), you may notice that almost all the cars were private. Most of the traffic flow is specialized materials for factories, and non-wood construction materials. Coal is moned in Alaska, pretty darned good stuff, too, but it's burned locally. I have no idea if the foodstuffs arrive in containers of boxcars; perhaps someone woth more frequent visits to Seattle can expound further.