Carl, Princess Donut, Katia, and Mongo arrive on the fourth floor, the Iron Tangle, inside a filthy moving subway car. Carl immediately focuses on basic survival problems: getting dressed, understanding the train layout, finding a map, and locating a stairwell or saferoom before the ten-day timer becomes a serious threat. At the same time, Donut is forced by Character Actor to choose a new class under a short deadline, and with Mordecai's help she becomes a Hooligan, gaining a constitution boost and the new Mascot skill that rewards the whole party when Mongo hurts or kills enemies.

As the train races onward, Mordecai explains that the fourth floor appears to be a giant rail network with settlements, shops, and inns at stations, and he warns the party not to leave the train except at transfer stations. He also realizes Donut may be benefiting from a system loophole: if she levels class-granted skills through use, she may keep some of that progress even after switching classes. Meanwhile, Katia uses Pathfinder to read the train map, revealing that they are in car 20 of a 20-car train and that several cars ahead are unusual, especially car 15, which appears on the map as an unknown area.

Carl leads the group forward through several empty passenger cars to investigate car 15 before the next stop. Just as they reach it, the train pulls into Sirin Station and Donut senses hundreds of level-6 Dreks, ravenous baby-like monsters, swarming the platform and the train doors. To avoid being overrun, Carl opens car 15 and the party dives inside, only to discover it is a dark, foul-smelling janitor car occupied by two level-17 Jikininki. Carl, Mongo, and the others kill both ghouls quickly, gaining the first benefit from Donut's Mascot skill, but the victory traps them between Dreks on both sides of the janitor car.

Carl searches for options while the monsters scratch at the doors. He decides the janitor car must be intended as a defensive choke point and begins improvising: he places an alarm trap and a spike trap in the forward gangway so the party will get warning and damage if the Dreks break through from that side. Carl then tries to access the floor panel beneath the gangway in hopes of finding train controls or electrical systems, but the service plate is magically locked and requires a Red Line Engineer's Key. At Mora Station, more enemies board nearby cars, including much larger monsters, and Carl is reminded how vulnerable he is when one Drek bites into his foot because his pedicure-based foot protection has expired.

When the alarm trap finally blares, Carl abandons the idea of holding the car and switches to escape. He realizes Protective Shell stays fixed where it is cast instead of traveling with him, so he casts it while the train is moving; the stationary barrier effectively becomes a wall that the speeding train drags mobs into, crushing Dreks and a larger Psycho Sticker through multiple cars. Carl then orders the party to retreat toward the rear, shutting doors as they pass through blood-smeared cars full of pulverized monsters and scattered loot. The chapter ends with the group still on the train, shaken but alive, looting gold and hurrying toward transfer station 83 in hopes of finally finding a saferoom.