The Christmas package holding the box moves between scenes. When Steve Zahn first removes the box the package is on the floor, the camera switches to a long shot, we see the package back on the man's lap and so forth throughout the scene of everyone picking up and looking into the box.
After Miles leaves to notify next station about the dead man, Peter approaches the corpse. On the close up you can see the chest moving because of breathing and not because of the movement of the train.
The policeman's pistol is a Czech CZ 9mm. American police officers do not carry Eastern bloc sidearms.
When the train is stopped by the police - during the whole time the policeman checks the passenger list - the filming inside the train is accompanied by train wheel sounds, suggesting it is moving (actually quite fast).
Chloe, the medical student, mispronounces carotid (as in carotid artery) as "CARE-o-tid," rather than the standard "kuh-ROT-id."