This entire ad is a homage, or parody if you like, of the Post Office's 25-minute documentary, "Night Mail", made in 1936. Poet W. H. Auden had written a verse specifically for that film, showing the scale of it all.
Here the opening verse is re-purposed as we see the northbound traveling post-office cars on British Rail.
Client: British Rail
Voice-over: Sir Tom Courtenay
Director: Hugh Hudson