Continuity mistake: When Poirot and Mr. Ratchett first meet in the restaurant car, Poirot has to correct Ratchett's pronunciation of his name. Poirot then points with his right hand toward Ratchett, but when it cuts to a close-up of Poirot, both his hands are on the table. The angle changes back, and Poirot's hand is once again lifted.
Continuity mistake: During the flashback to the murder, watch closely right after Mary Debenham stabs Mr. Ratchett. One moment she is in the room, the next she is back in the doorway.
Continuity mistake: Hercule Poirot takes the German maid to her cabin to look for photographs. She removes her suitcase from the shelf and opens it, showing a very obvious shot of a porter's tunic with many very shiny buttons and space for the missing button (evidence found earlier by Lauren Bacall). The shot cuts away and then immediately back, by which time a porter's hat has appeared on top of the tunic covering all of the buttons previously in shot.
Continuity mistake: When Mr. Hardman is interviewed by Poirot, he states "I'm not a theatrical agent," while gesturing with his right hand. The angle changes to show Mr. Hardman from the front, and both his arms are suddenly on the armrests of his chair.
Continuity mistake: In one of the flashback scenes, as the drugs start working on Mr. Ratchett, in a close-up he is holding the last anonymous letter in both hands. Ratchett then removes his glasses with his right hand, and places that hand on his forehead. Back to a close-up of the letter and the letter is in both hands, but when it cuts he is holding the letter with only his left hand again.
Continuity mistake: While it is fairly obvious that many of the flashback scenes are not the same as the original scenes one in particular stands out: during McQueen's second interrogation he describes Mrs Armstrong as being "gentle and frightened" while in the flashback he says "shallow and frightened".
Continuity mistake: Widmark's eyes are open when his body is discovered. But in the scene he is drugged he is last shown with them closed.
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