[[WMG: The sound of the bell is an ultra high sound.]]
As a person ages, they become unable to hear the most high-pitched sounds anymore. So that would explain the adults not hearing the sound of the bell. As for why the main character hears it even as an adult, let's say his hearing ability is superhuman.
* OOO! Good one!
* It's an interesting theory, but it doesn't explain why the Hero Boy ''couldn't'' hear it until after he decided that he believed in Santa Claus, and meant it. Or why, as he says, it rings for everyone else he knows that still truly believes.
[[WMG: The Hobo is Jesus]]
Perhaps he's a ghost, which could tie into the Holy Spirit. Perhaps he died, but now lives.
Jesus was an unshaven man who had no belongings, was disbelieved by many, and shared a lot. He also saved a lot of people. The Hobo has no belongings but the very basics for survival, and yet shares what belongings he has. Most people don't believe he exists. He's [[CaptainObvious unshaven and dirty, since he's a Hobo]]. He saved the main character by sharing his "joe" so the boy wouldn't freeze to death, and he saved the conductor.
--> '''Hobo:''' What exactly is... is your persuasion on the Big Man, since you brought him up?\\
'''The Boy:''' Well, I... I want to believe... but...\\
'''Hobo:''' But you don't want to be bamboozled. You don't want to be led down the primrose path! You don't want to be conned or duped. Have the wool pulled over your eyes. Hoodwinked! You don't want to be taken for a ride. Railroaded!\\
''[Hobo puts out fire with the joe] ''\\
'''Hobo:''' Seeing is believing. Am I right?
Possibly referencing the main character's agnosticism.
The Hobo understands, just as Jesus was filled with fear and doubt before his death (before accepting his fate). "Seeing is believing" refers to the Hobo having seen the "Big Man" (Santa; symbolically God); since the Hero ends up meeting Santa, it seems like the Hobo ends up preaching to him subtly (Parables) and leading him down the right path.
** The Hobo is a little too mean to be Jesus.
[[WMG: Billy is [[MalcolmInTheMiddle Dewey]].]]
The reason that Christmas "doesn't work out for him" is that Malcolm, Reese, and Francis have always ruined Christmas for him. Lois hasn't been shown to completely cancel Christmas before, but she's threatened it. Maybe in this MalcolmInTheMiddle / ThePolarExpress world, she HAS completely canceled Christmas. Besides, the two have an uncanny resemblance.
[[WMG: The Hobo is Santa Claus himself.]]
Santa can take on a completely different appearance, and he can be in two places at once. He appears first to the Boy as the Hobo because he knows that the Boy won't believe his true form early on, and is preparing him to believe. This is either weird or awesome, your call.
[[WMG: The movie takes place in the same universe as GalaxyExpress999]]
The North Pole is not on Earth, it's a different planet entirely. The fact that, for several hours, it's five minutes to midnight refers to North Pole time, which runs slower than Earth time.
[[WMG: A true visit from Santa is a once-in-a-lifetime experience.]]
This is why the poor boy never received a gift before the events of the movie, and why the parents don't wonder who puts the presents under the tree. He doesn't deliver to every house every year, but only to a lucky few. Most of the time, the parents put the gifts under the tree. The poor boy's parents didn't do this, so that's why he never got a present before his turn for a Santa gift came up.
[[WMG: The Hobo is Jack Frost]]
He claims to be the king of the North pole, he can turn into snow, he can be mischievous and kind of a dick at times but isn't actually a bad guy. Most of these traits seem to apply to Jack in other movies.
[[WMG: Not only is the Hobo a ghost, but the conductor, the engineers, and the guys who served the hot chocolate are ghosts as well.]]
[[WMG: The Conductor is Hero Boy grown up and sent back in time by Santa.]]
The Conductor has many, many moments when he acts as if he just knows whats going to happen, as if he's seen it before. Why else would he signal the train to leave Hero Boy and Billy? He already knew that they would still make it on.
Add to that, what if Hero Boy's wish to Santa was "to always believe"? Santa would make a grown up who believes his magical train conductor. Time manipulation is obviously at play in the film, why couldn't Santa wait for Hero Boy to grow up, possibly even die, then send him back in time to when the Polar Express came into existence?
This would also explain why the Conductor acts with such indifference to Hero Boy in most of their interactions. How would you treat your disbelieving younger self?
Finally, Tom Hanks is the motion-capture for Hero Boy, the motion-capture/voice of the Conductor and the voice of Hero Boy as an adult narrator at the beginning and end of the film.