Inspector Hornleigh Goes To It

Inspector Hornleigh Goes To It is a 1941 British detective film directed by Walter Forde and starring Gordon Harker, Alastair Sim, Phyllis Calvert and Edward Chapman. It was the third and final film adaptation of the Inspector Hornleigh stories.
It was released in America by 20th-Century Fox under the title Mail Train.
Plot summary.
Hornleigh and Sergeant Bingham join the army in an effort to uncover a ring of German spies.