Continuity mistake: About 3/4 through the film, which is supposedly set in the 1900s, there's an exterior shot of Jenny Agutter (as Roberta) talking to William Mervyn (as "the old gentleman"). Over Mervyn's shoulder can be seen a white delivery van driving rather fast along a road on the other side of the valley from where the filming took place (this is very obvious in the wide-screen version but less noticeable in the pan-and-scan TV version).
Revealing mistake: When the 3 children are waiting for their mother, the rain is pouring down. Yet, when they are gathered around the Russian guy on the floor, their clothes are dry.
Continuity mistake: When the Grammar School boys are on the paper chase they run through a railway tunnel. The Railway Children go to the opposite end to see the grammar school boys come out, as they are waiting for the final boy to come through, the paper that is thrown on the ground for the boys to chase appears then disappears several times.
Other mistake: In the last scene where the children are walking away from the house after their father has gone in, in the background on the right hand side of the screen, a tree falls.
Other mistake: We see the old gentleman travelling this route several times during the course of the film. However he is always going the same direction! Does he teleport the return leg? We know this is a two way line as trains are seen going the other way, including the train father returns on.
Revealing mistake: In the scene where the children are stopping the train with the red petticoats, just before the train has completely halted, you can see the reverse effect used in the film. The train is actually reversing away from them for safety, and when reversed, the red petticoats are swaying with unnatural movements as they're being waved by the children.
Continuity mistake: When the children stop the train, the red petticoat Phyllis is waving quickly changes position between the shots when she is waving it, and when she stops waving it and runs towards the driver.
Plot hole: It is clear that the tunnel through which the paper chase leads is straight and level. Anyone running from one end to the other will get there quickest by this route. However the three children confidently predict that by climbing up a hill and back down the other side they will get to the other end first, and succeed. This would not happen.