In one of the overhead model shots, the rear power-car of the train suddenly disappears.
When the helicopter first catches the tank in its harness, it is a Panzer. In distant shots, the tank changes to a Sherman, and then back to a Panzer in close-ups.
The "atomic-powered train" is actually a Blue Pullman, a diesel-electric multiple unit used on British Railways between 1967 and 1973.
When we see the phantom bagpiper on the battlements, you can clearly see it is Harper in a highlanders' suit. When we see him up close, he is wearing a mask.