The Polar Express is about faith, and the power of imagination to sustain faith. It's also about the desire to reside in a world where magic can happen, the kind of world we all believed in as children, but one that disappears as we grow older.
Santa is our culture's only mythic figure truly believed in by a large percentage of the population. It's a fact that most of the true believers are under eight years old, and that's a pity.