YouTube by definition is a commercial, for profit entity, so there is obviously no fair use of copyright materials. Putting a video on YouTube of your own Polar Express Lionel train running under your Christmas tree is highly unlikely to bring the copyright hounds down upon you, as stated. Incorporating actual footage from the film in your YouTube video would not be considered fair use. In general, anything that is considered mass media and commercial (like YouTube) is going to receive more scrutiny than what you do in your town hall atrium, obviously.
A public display in a bank, church or mall, with the Polar Express movie playing on a portable screen, with no admission charge, no payment to the layout displayer, perhaps with images of the movie on billboards, is likely to be considered fair use, assuming anyone would bother to care. So if that's what you have in mind, I wouldn't be concerned about the ethics or legality. The worst that could happen is they'll tell you to stop, since you are making no profit and causing no financial harm. If you are really concerned about this, you'll have to consult an intellectual property attorney, and that will likely not be free. Or you could just email the owner of the copyright and explicitly ask them what they will allow (not much probably). Hope that helps.