5. Grierson went on to head the GPO film unit in England in the 1930s and he became a major exponent of this poetic-realist approach to documentary. Nightmail (1936) began as an informational film about the mail train from London to Edinburgh but the filming and editing emphasised the poetic elements of film form: movement, rhythm, light and sound. Critics of Grierson accused him of neglecting the social and political issues in his films in favour of a modernist approach that celebrated machinery more than human beings.