The Major and the Minor (1942)

Susan Applegate, tired of New York after one year and twenty-five jobs, decides to return to her home town in Iowa. Discovering she hasn't enough money for the train fare, Susan disguises herself as a twelve-year-old and travels for half the price. Caught out by the conductors, she hides in the compartment of Major Philip Kirby, a military school instructor who takes the "child" under his wing.