are the characters well developed?

This is a subjective question. I think they were developed enough for the context and genre of the story. Before the boy comes across the eponymous Polar Express train, he is struggling to decide whether or not he believes in Santa Claus. The train, which is a metaphor for the boy's journey of self-discovery, takes the boy and a number of other kids (some of whom also don't believe in Christmas and Santa Claus) to Santa's village where he creates gifts for people across the world.