Night Mail is a 1936 British documentary film produced by the General Post Office (GPO) Film Unit. The 24-minute film documents the nightly postal train operated by the London, Midland and Scottish Railway (LMS) from London to Scotland and the staff who operate it. Narrated by John Grierson and Stuart Legg. The film ends with a "verse commentary" written by W. H. Auden. One of the locomotives featured in the film is LMS Royal Scot Class no. 6115, 'Scots Guardsman'.

Wikipedia; https://en.wikipedia.org/wiki/Night_Mail