This film, along with the equally allegorical High Noon (1952), was a deciding factor in making Howard Hawks create Rio Bravo (1959), a return to more optimistic, less revisionist Westerns.
When Charlie comes to the saloon to warn Ben that lawmen are coming, he places his hands on the top of the doors. It then cuts to an exterior view of the doors, and Charlie's hands are not visible.
The steam locomotive pulling the train to Yuma is clearly of early 1900's vintage. The engine has a steel cab, electric headlight, and other applications that were not available or in common practice in the 1880's.