Carl and Donut have survived so far, but this fourth level is unlike anything they could imagine. The Iron Tangle: an impossibly complicated subway system tied together into a knot of trains of all kinds. The top ten list of most popular crawlers is now confirmed, and Carl and Donut have made it.

But that popularity comes with a price . . .

They each now have a bounty on their head. Carl and Donut have survived so far, but this fourth level is unlike anything they could imagine. The Iron Tangle: an impossibly complicated subway system tied together into a knot of trains of all kinds.

The top ten list of most popular crawlers is now confirmed, and Carl and Donut have made it. But that popularity comes with a price . .

. They each now have a bounty on their head. To get through this level, they must solve the puzzle of the floor.

The secret to unravelling it all may be hidden in the pages of a seemingly useless book. But if they want to come close to success, they'll have to work with other crawlers. Making friends is easy…until they want to kill you.