From the back of the book:
Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they're going on a train trip across the country with their Aunt Jane!
The Boxcar Children love exploring the train, watching the beautiful scenery from the observation car, and eating in the dining car. But the train ride gets really interesting when they meet a young woman who has had something very valuable stolen from her. Can the Aldens solve the mystery before the train pulls into the last stop?