Summary from Goodreads:
The Boundless, the greatest train ever built, is on its maiden voyage across the country, and first-class passenger Will Everett is about to embark on the adventure of his life! When Will ends up in possession of the key to a train car containing priceless treasures, he becomes the target of sinister figures from his past. In order to survive, Will must join a traveling circus, enlisting the aid of Mr. Dorian, the ringmaster and leader of the troupe, and Maren, a girl his age who is an expert escape artist. With villains fast on their heels, can Will and Maren reach Will’s father and save The Boundless before someone winds up dead?