The floor's five-hour countdown to collapse, running as Carl launches the train into the abyss.
The chapter opens with five hours left on the level collapse timer. Within that window Carl positioned the Interdiction Rapid-Response Railway Repair Cart on the employee track, rigged The Nightmare Express, sent it through the portal into The Abyss, and brought down the floor's ghoul system.
With the ghouls broken, the stairwell at Station 36 stood open and Daniel Bautista's group could finally move in toward the stairwells. Nobody descended until Carl returned; then Elle McGib, Agatha and finally Carl's party went down, leaving the collapsing Iron Tangle behind.
The deadline that forces the timing of the entire floor-ending operation.
Also appears in books you've already read — spoiler-safe to their endings: