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.
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.