"Frank" and "Ollie", the two trainmen that Kent interviews after the derailment, are caricatures of Disney classic animators Frank Thomas and Ollie Johnston. They also perform the voices for the characters. Animation writer Earl Kress has said "that Frank and Ollie are also life-long train enthusiasts and have extensive scale model railroads in their backyards like Walt Disney used to have."
In order to better blend the computer-generated Giant into the traditionally animated film, technicians came up with a program that gave the Giant's lines a slight "wobble", in order to match the natural line imperfections found in hand-drawn animation.
In Guardians of the Galaxy (2014) you can notice that the voice of Groot is very similar to the voice of the Iron Giant. That is because they are both voiced by Vin Diesel and in both times the sound technician that worked on it was Doc Kane.
Was originally meant to be a musical. Pete Townshend and Des McAnuff developed it as a stage musical, using songs from Pete Townshend's concept album "The Iron Man" (much like the stage version of The Who's Tommy (1975)). Des McAnuff decided it would work better as an animated feature and pitched it to Warner Bros.
Dean's newspaper in the diner scene had an ad for RINSO, THE GRANULATED SOAP clothing detergent which was a very accurate ad (and image) of the time, down to the silhouettes of "happy home-makers" on the box.
When the Giant's hand is "watching" TV, the clip that was to be playing would have been the opening to Walt Disney's Wonderful World of Color (1954), but the filmmakers couldn't secure the rights, and substituted the Maypo commercial instead.
The newspaper headline that Dean McCoppin is reading ("Disaster Seen as Catastrophe Looms") is similar to the headline that Jim Dear is reading in Lady and the Tramp (1955) and that Jiminy Cricket is reading in Fun & Fancy Free (1947).
Among the comics Hogarth shows the Giant is an issue of "The Spirit". Since the comic-book version ended in 1950 ("The Spirit" was a newspaper comic originally), it's unlikely that, in real life, Hogarth would have had any issues. But before Brad Bird made "Iron Giant", he tried (but failed) to get backing for a "Spirit" animated film.
In the scene where Kent is using the Hughes' phone, the humorous oven mitt hanging on the wall beside him appears to be a stylized version of the "family dog" from Brad Bird's animated short of the same title which appeared on Amazing Stories (1985). (Season 2, Episode 16: Family Dog, 16 February 1987).
Cloris Leachman's character Mrs. Tensedge originally appeared in another scene of the movie as well as more dialog according to a deleted pencil test scene. The scene ended up being removed from the story due to time restraint. In the film she only said 7 words.
Dean's newspaper in the diner scene had an ad for BATTERY OPERATED MOON EXPLORER, which was a replica of the tin toy robots box front of the time. The same ad is mysteriously replaced by an ad for a lounge chair the next time the open newspaper page is shown.
The trivia item below may give away important plot points.
The Iron Giant (1999) roughly takes place on a five day period (Wednesday through Sunday) On Wednesday night, the giant crash lands on Earth. After school the next day, Hogarth visits his mom at the diner and that night he encounters the giant. It is now Friday and Hogarth goes to school and afterwards goes and finds the giant. That night he meets Kent and then Hogarth and the giant find themselves at Dean's junk yard where the giant has food and a place to rest. Saturday is when Kent questions Hogarth all morning. Hogarth visits Dean and the giant that afternoon and go to the lake and witness the death of the deer. At night, Kent interrogates Hogarth and informs the army of the giant's existence. The army arrives in the morning and fights the giant and then the giant saves everyone.