When the car is left on the tracks, and the train hits it and breaks it apart, as it flies apart it is clearly just painted wooden sections fastened together on a frame to look like a car. There is no engine or drive train or interior, no seats or glass, and the wooden pieces can clearly be seen.
The train worker carrying the shotgun fires that shotgun at least ten times during scene in the tunnel where everything goes black. The sound effect for the shotgun had been established in previous scenes. The shots were far to close together to allow for any reloading; and since he was carrying a double barrel shotgun that many shots would have not been possible.
When Flagg and McKay are running along the roof of the train, the view from inside the train shows their shadows on the cliff wall beside the tracks level with their actual position on the roof, as though the sun were nearly on the horizon throwing their shadow straight onto the cliff wall directly behind them. But when the view switches to overhead on top of the train, their shadows are directly below them as though the sun were directly overhead. This same pattern is repeated several times, camera overhead showing their shadow directly below them, then inside showing their shadows on the cliff wall out to the side.
When Martin Balsam is getting dressed after being interrupted with his dalliance with Tina Louise, his collar is unbuttoned and vertical. When he is wearing his dressing gown, the collar is covered by the gown. However when he steps outside the collar is visible again.