If you don't know the plot: The '3:10 to Yuma' is a train route, like if I were take the '5:30 to New York', or the '2:45 to Cleveland'. A well-known, feared bank robber (Ben Wade) is caught. Again.
The marshals need someone to escort him to the train station to board him onto the 3:10 to Yuma, to be imprisoned in Yuma and eventually hung in public. A very, very...