According to a 1930s Great Western Railway timetable, The Plymouth Express did not go anywhere near Bristol.
The train on the main Great Western line (Paddington to Bristol) is depicted more than once running on a single track.
The story is set in the 1930s, yet the train carriages at Paddington station are of the BR MK1 type not built until 1951; and as the steam train pulls into Plymouth station, several 1980s-built diesel trains can clearly be seen in the background.