The 'junk' arrival-souvenir hat revealed as a golden ticket — equipping it lets a crawler pass the Abyss portal to the stairwells.
The Train Conductor's Souvenir Hat — the blue-and-white toddler cap embroidered 'I rode the Iron Tangle,' handed to every crawler on arrival and long dismissed by Carl as non-magical junk piling up in Princess-Donut's hoard — turns out to be the floor's secret golden ticket. Analyzing the Abyss portals with his Subspace-Portal-Analysis, Carl finds three of his items 'marked' as able to pass: the Ochre key, the Steam key, and 'that stupid fucking train conductor hat.' Simply equipping the hat and riding a colored train to the end of the line teleports the wearer through — though the train car doesn't come along, dumping them into the zombie-filled engine-car trainyard near the stairwells. Carl broadcasts this to his whole network, and crawlers who'd been selling their hats for 5,000 gold suddenly realize what they nearly threw away.
A small blue-and-white toddler-style cap embroidered 'I rode the Iron Tangle' — outwardly worthless.
The overlooked arrival trinket that is secretly the simplest escape key off the train floor.