The proposed rail link between the continental US and Alaska would be part of a larger passenger rail network spreading across the US. Now, I will admit I am biased more towards railroads as a means of transporting human capital, with that being said, I think that there would be plenty of benefits to building a rail link between Alaska and the Lower 48, such as increased trade between northern Alaska and the rest of North America, as well as Human Capital being able to move more freely between major cities along the west coast of the United States. This would allow for more skilled workers to easily take residence in major Alaskan cities, and using their talents to develop the Alaskan economy more.
I do, however see one potential problem with the proposal. Now, I may be wrong, but I seem to remember that between Alaska and the lower 48 is a tract of land that *GASP* may not actually belong to the United States (hard as that might be to comprehend). You know that empty pink area on your school maps that looks like America's sombrero? I think that might actually be a sovereign nation of some description.