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).
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.
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.