Wikisimpsons - The Simpsons Wiki
The Polar Express is a train used to transport people to the North Pole.
The Simpson family met a conductor on their train who said they were on the Polar Express. Bart then said that he skimmed through the book and skipped through the movie on DVD, and found out that the Polar Express was just a dream. The conductor told him "Then I guess you're out of luck!", and they ended up on the side of the train track.
When the train was going to the North Pole, Otto fuelled it with cannabis, creating weird illusions as it was flying. It reached the North Pole and dropped Bart off.
On the return journey, a police car was chasing it and Otto jumped out of the train, leaving it to Bart.
 Behind the Laughter