Sally Thomsett was twenty when she was cast as eleven-year-old Phyllis. Her contract forbade her to reveal her true age during the making of the film and she was not allowed to be seen smoking, drinking, going out with her boyfriend or driving the sports cars that were her passion. Even the film crew did not know her true age.
Lionel Jeffries grounded Jenny Agutter and Sally Thomsett when he caught them in a nightclub in Leeds after they had sneaked off the set for a night on the town.
Sally Thomsett, who plays Phyllis, the younger sister, is actually two years older than Jenny Agutter who plays big sister Bobbie.
The British Film Institute ranks this film as the 66th Best British Film of all time, the only children's film to make the list.
The late Bob Cryer, his wife Ann Cryer and son John are all extras in the filming on the Keighley & Worth Valley Railway. Labour MP Bob Cryer was a prime mover in restoring the historic railway, and plays the train guard. In the final scene, Bernard Cribbins can be heard to say, "Right away, Mr Cryer," before the train pulls out. Likewise, Graham Mitchell, another volunteer, plays a train guard in other scenes and Bernard Cribbins again refers to him by his real name.