First, my disclosure: I didn't read this book for the first time as a child.
The story is a straightforward tale about a boy who is right at that age between believing in Santa Claus and not believing. On Christmas Eve he is whisked away to the North Pole on the Polar Express and meets Santa who gives him a gift. The gift is a reminder about the joy of Christmas for all those who truly believe.
The artwork is exceptional for the tone of the story, which is to say it evokes a Norman Rockwell feeling of longing for the classic childhood. The gift of the bell seems to represent the boy's faltering belief in Santa Clause. Sometimes it's strong; sometimes it's weak. By the end, only those who truly believe in the spirit of Christmas can hear the bell.
I appreciate the childlike atmosphere of Christmas and how simple and good the world feels every December. I don't agree with the book's notion of aging being some sort of an automatic loss of the Christmas spirit. An experienced understanding of Christmas can be the truest of all. If anything, the bell should ring louder for the adults.
The Polar Express will no doubt endure as one of the best stories to read to your child each Christmas even though its message seems outdated.