Revealed as a bigoted galaxy-map political cartoon: the named lines spell the Syndicate logo, and every line hides a mirrored, upside-down twin.
Mordecai and Katia Grim finally decode what the Iron Tangle really IS. Mordecai recognizes the named train lines tracing the Syndicate's overlapping-rings wormhole logo, with the abyss as the galaxy's center and the tracks as the original worm-paths; the whole floor is a racist political cartoon smearing the Krakaren collective ('the Apothecary') and the goat-like Plenty who invented the tunneling comms system, with the station mimics standing in for the extinct predator-race the H'lene. Katia goes further: each line is a double-sided 'noodle,' its outer track shadowed by a mirrored, UPSIDE-DOWN twin line below, with an empty chamber between — which is why there are only six giant mimics (each occupies two stations at once) and hints the floor may literally flip.
Mordecai recalls an old fourth floor of sliding puzzle-pieces that assembled the Syndicate logo, then spun on its axis to dump everyone into a hidden chamber below. The Tangle is a subway/rail labyrinth spanning colored lines (zomp, Vermillion, Sinopia, Grullo, Mindaro), named loop-lines (Nightmare, Escape Velocity), trainyards, and stairwell stations — all now understood as one bigoted map of the galaxy hiding a lethal final trap.
A vast, chaotically interwoven subway/rail network of colored and named lines twisting over and under each other around a central abyss, dotted with trainyards, transit stations and circular stairwell stations.
The dungeon's fourth floor: a train-labyrinth designed as a galaxy-map political cartoon with a hidden inverted lower layer.
Also appears in books you've already read — spoiler-safe to their endings: