The Merch: Lionel's very awesome Polar Express model-train set. It also comes with a replica of Santa's sleigh-bell.
The Other Darrin: Andre Sogliuzzo did the voices for Smokey and Steamer except in the Caribou scene, where Smokey (the fat one) was voiced by Michael Jeter, the actor who did the motion capture for both characters. This may have been left in as a tribute to Jeter, who died during post-production.
Averted with Tom Hanks. He does most of the voice acting, but he doesn't voice any characters who have to talk to each other. You could almost call it "Acting With Himself", since Hanks did the motion capture for Hero Boy as well as for the conductor, hobo and Santa, just not HB's voice.
Played straight with the Smokey the engineer and Steamer the fireman, with the exception of the aforementioned Caribou scene where Smokey's VA temporarily changes.
Write What You Know: The main protagonist is from Grand Rapids, Michigan, also the hometown of the book's author. In one scene, the train passes by Herpolsheimer's, which was (until the 1980s) a Grand Rapids department store that was known for its lavish Christmas displays.