PhilK:
Escaping from the train guard, she hides in the compartment of a Major, played by Ray Milland, who improbably enough buys into her kiddie impersonation.
They made it more probably by giving him a reason not to see well. If memory serves, he really needed glasses, but he wasn't wearing them to strengthen his eyes so he could be deployed in the field.
In short, only a blind man would mistake the grown woman for a little girl, but the Major was at least half blind.
why do I know so much about this old movie? Because I used it as the basis for a story where a university student borrows the concept to get a child's fare on a bus ride home for the holidays.