The Iron Tangle: an impossibly complicated subway system tied together into a knot of trains of all kinds. The top ten list is populated, and Carl and Donut have made it. But that popularity comes with a price. They each now have a bounty on their head.They have to work with other crawlers to solve the puzzle of the floor, but how can they do that when they don’t know who to trust? The secret to unraveling it all may be hidden in the pages of a seemingly-useless book. Welcome, Crawlers. Welcome to thefourth floor of the dungeon