The Major and the Minor (1942)

Susan, a New York working girl, is desperate to return home to Iowa but lacks the full fare for the train. She disguises herself as a 12-year-old to travel at half price. When the conductors grow suspicious, she hides in the compartment of Major Kirby, who believes she's a child. The two develop a bond, but his fiancée grows suspicious. As Susan navigates this deception, she falls for the Major, while his fiancée tries to sabotage his future.