Mary is thrilled to learn that she has qualified for a state-wide mathematics competition, but the excitement is halted when Charles cannot afford the expenses for the trip. When Mary silently forfeits, the school board recognizes the problem and agrees to finance the whole thing. Caroline accompanies Mary on the trip, while Laura steps in at home to care for Charles and Carrie.

Mary, Laura, and Mr. Edwards' adopted son, Carl, long to get a first-hand look at the kinds of trains they learn about in school. The kids eventually con their parents into letting them go on a business trip with them. While messing around on a caboose at the train station, the children are in for the ride of their lives when it detaches from the train and runs loose.