When the Hero Boy first meets the Hobo on the roof of the train, he is playing the carol "Good King Wenceslas." The story of Saint Wenceslas I, Duke of Bohemia is that of a king braving the harsh winter to bring alms to the poor on the Feast of Stephan, December 26th. His page finds he can't go on through the harsh conditions and is directed to walk in the footprints that the king has made in the snow. The Hobo directs Hero Boy to follow behind him and ultimately helps him reach the engine before they make it to the tunnel, thus allowing him to find and help his friend. This is also a representation of the Holy Spirit idea of the Hobo, that he guides those who believe in Him to safety, even in perilous times.
The film used 3D motion capture techniques to digitally record the physical performances of the actors before "skinning" them with their animated forms. All the children's roles were acted by adults using oversized props to get the movement right.
In the scene where Smokey and Steamer (the fat man and the red-bearded man) are trying to catch the pin, a flux capacitor (from Back to the Future (1985), another Robert Zemeckis film) can be seen for a brief moment.
The real name of the Hero Boy is never mentioned in the film. However, according to books containing information about The Polar Express, including art books and fact books, the Hero Boy's name is Chris, named after the novel's author, Chris Van Allsburg.
The soldier doll that the Hero Boy is playing with on Christmas morning is part of the recycled toy program spoken of by the conductor. You can see the same toy as a puppet in the background of the scene with the Scrooge puppet.
A close examination of all the ticket numbers reveals that they all contain the number "1225" in them. Pere Marquette No. 1225 was used as a model for the Polar Express locomotive. 12-25 also refers to the date of Christmas - December 25th.
The locomotive in the movie is based on the Pere Marquette 1225, a restored steam locomotive located in Owosso, MI. In fact, many of the sound effects of the film's train are recordings of the actual train. The train is often run between Owosso and nearby Ashley during the holiday season.
The close shots of Hero Girl's train ticket floating in the air are a nod to the trademark shots of a feather doing the same in Robert Zemeckis' earlier film, Forrest Gump (1994), which also starred Tom Hanks.
In the beginning of the movie, Hero Boy looks at a picture of himself on a store Santa's lap. The store's name on the photo is Herpolsheimer's, an old department store in Grand Rapids, Chris Van Allsburg's hometown. Later the train passes by the store.
The Hero Boy has a University of Michigan pennant (featuring the well-known maize-colored "block M") on the wall behind his bed. Chris Van Allsburg is a real-life graduate of the University. Additionally, Pere Marquette 1225, the locomotive used as the basis for the one in the film was donated to the Michigan State University after being retired from active service and displayed on the campus from 1957 to 1983.
The track level view of the locomotive's cow catcher coming to a halt right at the camera (when the emergency brake is applied to let Billy get on the train at 0:14:05) is a repeat of the same view in Back to the Future Part III (1990), when Clara is leaving and applies the emergency brake to stop the train when she overhears about Emmett Brown's heartbreak (at 1:25:53).
Hero Boy pulls the cord blowing the train whistle and says he has wanted to do that all his life. Emmett "Doc" Brown (played by Christopher Lloyd) said the very same thing when he blew a train whistle in Back to the Future Part III (1990), also directed by Robert Zemeckis.
The trivia items below may give away important plot points.
The scene in the North Pole City communications room features an elf describing a bad little boy in New Jersey named Steven who is terrorizing his two little sisters. This line is a nod to Robert Zemeckis' friend and mentor, Steven Spielberg. Spielberg grew up in New Jersey and has admitted many times that he frequently terrorized his two younger sisters.
Tom Hanks provides the voice for many characters, even if his likeness is discernible only as the Conductor. He voices, among others, both Santa Claus and the mysterious Hobo. The Hobo is merely suggested as being a spirit especially when he asks Hero Boy if he believes in ghosts. A deleted scene verifies that Hobo is indeed a ghost. The multiple roles of Tom Hanks are not simply an artistic choice, but they intentionally constitute some symbolic undertones in the plot. Santa Claus, the Conductor and the Hobo represent the Holy Trinity and the trip to the North Pole is a test of the Hero Boy's faith since he expresses some doubts about the existence of Santa Claus, who stands for God. The Conductor symbolizes Christ who continually tries to keep the children inside the train and ensures that the locomotive will not deviate (meaning that he guides the Christians on the right path). Finally the Hobo is the incarnate of the Holy Ghost in reference to the fact that he is a spirit who guides the Hero Boy continually testing his faith, and giving him options.
In the beginning scenes of the movie, particularly in the Hero Boy's bedroom, there are two closeups of a chrome hubcap with a baseball laying next to it. If you look closely during the second closeup of the hubcap as the boy's parents leave his room for the night after checking in on him, you can see a small reflection of the Polar Express train in the center portion of the hubcap.
The visuals for North Pole City are based on the architecture of the Pullman plant (the company that made railroad cars) in Chicago, Illinois, near the childhood home of Robert Zemeckis in the Roseland neighborhood. The clock tower, of which Santa comes out, is based on the Pullman factory clock tower and many of the other buildings are based on Pullman's unique architecture.
The Polar Express is another Christmas Carol movie. Hero Boy is Scrooge and doesn't believe in Santa anymore causing the three ghosts of Christmas (the Hobo, Conductor, and Santa) to change him forever. The Hobo is the Ghost of Christmas Past, and plays the role of Christmas Future. He tries to intimidate Hero Boy into changing his ways using only himself instead of the future. The Conductor is the Ghost Christmas Present and is in charge of keeping order in the present to get Hero Boy to Santa. Finally, Santa is Christmas Future and changes Hero Boy forever, like Scrooge was. They even pay homage to Dickens' classic by having a Scrooge puppet dance on screen briefly. (Controlled by Christmas Past)