The 15:17 to Paris

In the trailer for the movie The 15:17 to Paris, shortly after a hostess serves drinks to three young American men on a train heading for Paris, a man comes running through, looking panicked. Walking slowly behind him is a man carrying a rifle. We then go back in time to see the lives of the three American men, from their childhoods to their time training in the U.S. Army. In the last scene, one of the American men heads towards the man with the gun, who turns and points it at him.