The Major and the Minor (1942)

Susan Applegate (Ginger Rogers) decides to leave New York City and take a train back to Iowa, but she has only enough money for a child's ticket. She disguises herself as a young girl and, after being discovered by the train conductor, hides out in the car of Major Kirby (Ray Milland). Kirby believes she is a child and watches after her. But as Kirby's fiancée (Rita Johnson) and others grow suspicious of Susan's ruse, her cleverness is thoroughly tested.