The sound recordists equipment was unable to record a realistic sound of the mail train clattering over the joints in the track during the "two bridges and 45 beats" trackside mailbag collection sequence. Eventually they resorted to recording the sound of a model train being pushed back and forth over joints in a model railway track in time to the film of the man on the train counting the beats.
The film was shown on the British TV channel 'Talking Pictures' on 18th December 2018 and immediately prior to the start, the channel showed a card informing viewers that it had classified the film as 'PG' even though the BBFC had given it a U certificate (without cuts) as long ago as 1936. The reason for this is unclear.
The actual BBFC 'U' certificate from 1936 was then shown on the screen, a very rare occurrence for any British TV channel.
The shots of the interior of the carriage where the mail is sorted were filmed in a studio. An impression of movement was given by gently swinging the string that was hanging down from the top of the sorting boxes before each shot was filmed and telling the postal workers to walk with a rolling gait.
There's a shot in a station where the locomotive is being changed over so that two are being used in the journey from London to Scotland but it appears that there are more than two engines pulling the train.