The Mystery on the Train (The Boxcar Children Mysteries)

The Mystery on the Train (The Boxcar Children Mysteries) series created by Gertrude Chandler Warner

The children's aunt is taking them on a train ride from the east coast to San Francisco. They will spend some time there, then fly back. Before getting on the train, the see a teenager and a woman fight over an are portfolio. They later meet the girl. She seems to change details of her story each time. They learn that the portfolio is actually movie posters. Her relative that she is going to meet in California has promised the posters to somebody and is an scheme to steal them. They help rescue the posters and the girl decides she wants to go back to the east coast rather than spend time with relative in San Francisco. It is a decent kid sleuth story.