The man on top of the train in Polar Express is significant because he represents the conductor, who is responsible for leading the train and its passengers to their destination. The conductor is a key figure in the story, and his presence on top of the train helps to create a sense of excitement and anticipation for the journey ahead. Additionally, the man on top of the train is a visual representation of the holiday spirit that infuses the entire story. He is jolly and cheerful, and his presence helps to set the tone for an enjoyable and festive journey.