Tokyo-Sapporo shinkansen service should make a pretty good dent once it gets going. Lots of people take the shinkansen between Tokyo and Kyushu, even though flying is generally faster and cheaper for that distance.

Wasn't there discussion at some point about doing an "auto train" through the Seikan Tunnel? A Shinkansen that took cars to and from Hokkaido could be really popular during the holiday season.