The series is set on a seemingly infinite train traveling through a barren landscape in an alternate dimension. Every car on the train functions as it's own mini-universe which each have their own rules and laws of physics and contain a variety of bizarre and fantastical environments and creatures. The passengers on the train are people it picks up from the real world who have unresolved emotional issues or trauma. As the passengers traverse the train's cars, they have adventures solving challenges and meeting denizens who help them confront and resolve their past trauma and emotional problems. Their progress is represented by a glowing green number on their hand that goes down if they successfully confront and resolve these issues or goes up if they fail or backtrack on their progress. Once they resolve their issues, their number reaches zero and a portal opens up allowing them to leave the train and return home. Each season (referred to as a "book") follows a different character or group of characters during their journey of emotional healing and self discovery on the train.