Continuity mistake: When Richard Pryor is ramming the police cars after saying "demolition derby", as the shot changes he is not wearing any glasses then he's wearing sunglasses, next shot no glasses, then sunglasses again, then back to none.
Continuity mistake: At the end, when the runaway train is going through the garage, you see it approaching the end of the line as indicated by a large "bumper" at the end of the track yet when the shot changes to a front view to see it crashing through, the large "bumper" is gone.
Continuity mistake: At the beginning of the film, when Ned Beatty is hitting on Jill Clayburgh in the bar car, the drink in her glass refills itself from half-full to nearly full. (00:06:35)
Continuity mistake: When George puts on Grover's purple jacket in the men's room, the collar is turned up in back. In the very next shot, it's rolled neatly down again, even though he hasn't touched it. (01:10:00)
Continuity mistake: Gene Wilder's character has uncoupled the passenger cars from the run away engine. Richard Pryor, who was holding onto Wilder from the front cars, jumps from the runaway cars to the disconnected cars. If you look closely, you can see a stage hand's arm giving Pryor a push.
Continuity mistake: In the washroom scene, in between Gene Wilder saying "That's humorous" and "May I speak", Richard Prior is applying the shoe polish on Gene Wilder's forehead and not the nose, but next shot facing Wilder, there is no polish on Wilder's forehead but there is polish on his nose. (01:14:25)
Continuity mistake: When Devereaux is shot by the cop in the helicopter, he's alone in the Silver Streak's engine compartment, leaning out the open door. He's then hit by an oncoming engine. In later shots of the runaway train, there's no sign of his body or of any blood, and the door has somehow closed itself. (01:44:00)
Continuity mistake: While the train crash scene takes place at the fictitious Chicago Central Station, it was filmed at Toronto's Union Station in a discontinuous sequence. The train enters the station shed from the west, followed by a cab view filmed at the east end of the station. Finally, an exterior shot of the train inside the shed is shown, once again at the west end.
Continuity mistake: When Caldwell and Hilly get up from the table in the buffet car, she picks up the glass which has some champagne in it, but the shot of them walking up the aisle to leave the car shows Hilly with no glass, and Caldwell with two glasses which are empty. (00:13:53)
Continuity mistake: Grover is holding the pistol in his right hand as he points it at Devereau, an instant cut later and the gun is in his left hand.
Continuity mistake: In the train crash sequence, the real locomotive's headlamp has two lights side by side. However, when the movie cuts to the model crashing into the terminal, the headlamp has a single light. The model also lacks grilles on either lower face which were present on the real locomotive. (01:48:00)
Answer: Most likely it was a production decision. Having multiple extra actors continually smoking during multiple takes and over hours of filming would make for a smokey set, affect breathing and burn peoples' eyes, be a fire hazard, and create consistency problems with cigarettes being in various stages of use (freshly lit or almost down to a stub).
raywest ★