"Well, considering the fact that we have lost communication with the engineer, we're standing totally exposed on the front of the locomotive, the train appears to be accelerating uncontrollably, and we are rapidly approaching Glacier Gulch, which just happens to be the steepest downhill grade in the world, I suggest we all hold on TIGHTLY!" — The Conductor before the train approaches Glacier Gulch
Glacier Gulch, with a 179° grade, is the steepest downhill grade in the world. The Polar Express once sped down this roller-coaster-like track at a too great of a speed after Steamer broke the engine's throttle cotter pin, causing the train to speed out of control.
Before the train does so, the Conductor uses the ropes from Hero Boy's robe to tie him and Hero Girl onto the safety bar so that they do not fall off and tells them to hold onto the bar. After the train reaches the bottom of the gulch, it speeds over a roller-coaster like bridge. While that happens, the pin gets freed from the vent it fell into and Steamer catches it in his mouth, only to swallow it after the train goes over another hill. The train then ends up on the Ice Lake.
Later, when Hero Boy, Hero Girl and the Conductor get back to the passenger cars, Know-It-All tries explaining the events to Hero Boy, thinking he missed the whole thing.
The real steepest downhill grade in the world is on Baldwin Street in North East Valley, Dunedin, New Zealand with a maximum grade of 35% (19.29°).
On rail, it is in Calçada de São Francisco on the tramway network in Lisbon, Portugal, which has a maximum grade of 13.8% (7.86°).
In the film, there is a sign warning oncoming trains about the gulch having a 179° grade. However, a vertial drop is 90°, so it would make more sense for it to say 89°.