Harry Potter and the Chamber of Secrets - Two Hogwarts Expresses?
Near the beginning of the movie, when the Weasleys and Harry are trying to get into Platform 9 3/4, if you look the their left, there's a train waiting on the tracks. Look at the number on the front of the train and memorize it or write it down. Then, when Harry and Ron are in the car, and directly in front of the train, you look at the number on THAT train. It's the same one!