13-year-old Tulip finds herself on a mysterious train with an endless number of cars, each one its own universe, and must find a way to get home.

Mirror Tulip (M.T.), the mirror version of Tulip first seen in Book One's "The Chrome Car," wanders the train both to avoid the Mirror Police and to find a purpose. She stumbles upon a people-pleaser passenger named Jesse and a deer with mysterious powers.

Grace and Simon are the leaders of the Apex, an anarchic group of kids who believe they are entitled to the train. During one of their destructive missions, Grace and Simon get separated from the group and must find their way back. As the duo navigate through the myriad worlds of the train, they meet Hazel, an optimistic young girl, and Tuba, her gentle gorilla companion. Will Grace and Simon find new inspiration in Hazel’s innocence, or will the ways of the Apex recruit yet another aboard the train?

Childhood friends with opposing personalities, Mingi and Ryan, dream of becoming rock stars but find themselves on the Infinity Train. They explore the locomotive's inner puzzles — and their own relationship — in order to return home, together.