It’s the 25th anniversary of the original Mission: Impossible and a brand new Blu-ray release celebrating that includes a sit down interview with Tom Cruise conducted by current Mission: Impossible writer/director Christopher McQuarrie.
They even started mapping out how such a sequence would be filmed before figuring out how any of it would fit in the actual movie.
In the interview Cruise reveals that director Brian de Palma had the idea to do a sequence on a train before they had put together any sort of plot for the film.
Tom Cruise on Coming Up with Famous Mission: Impossible Scenes with Director Brian De Palma