Probably be easier to build the entire thing to the Russian gauge.
There was once a considerable network of 3-foot lines in Colorado, New Mexico, Nevada, Utah and Arizona. For a while they even handled through cars from the rest of North America, by one of two mechanisms: either the wheels were able to be slid in and out on their axles, or the trucks themselves (bogeys in european terminology) were switched out by lifting the cars right off them with a crane. The standard gauge cars swayed a bit, but (mostly) stayed on the rails, if not loaded too top-heavy.
Since the Alaska Railroad is already isolated from the rest of the North American rail network except by boat, it might be easiest to regauge the entire thing. You could even install the truck-switching mechanism right there on the ferry, and regauge the cars while in transit.
(Would such a line be electrified? The tunnel itself probably would have to be, but even for the rest I don't see conventional diesel locomotives liking Arctic weather all that much.)