The loco being passed as Thunderbolt (Lion) is entering the milk dock at Bristol Temple Meads (Mallingford) is a 'Star' class 4056 Princess Margaret, built July 1st 1914 and 'called home' October 1957. Stars had Castle-type outside steam pipes when they were given new cylinders. I saw this film on general release (in Watford) in 1953. Twenty years later I was living in North Somerset and remember the opening shot of Lion going under the S&D at Midford, which line I travelled over several times.