Other mistake: In the original version of the film there's a mistake in the opening credits where Colin Blakely's name is misspelled as Colin Blankey.
Other mistake: The opening montage shows the news story of the kidnapping of the Armstrong child unfold through newspaper articles. The kidnapping takes place in April 1930, but next to the headline of The Police Gazette about the getaway car, you can read a headline (incidentally, from a magazine with the name of the victim in place of the publication's name) that says "Snow-covered Rocky Mountain states feel first winter blasts." First winter blasts, in April. (00:03:45)
Chosen answer: They planted this red herring not to divert Poirot's attention away from them - they were the only passengers on the train - they wanted to divert him from the fact that they were ALL involved in the murder, because they all had a common bond with the child whom the victim murdered. Each one made out like they didn't really know anyone else on the train, but they were all in on it.
Kimberly Klaus