A subway line shaped like an upside-down fishhook — the party's main hunting-and-mapping route, home to conductor Vernon.
The Yellow Line is the subway route Carl's party rides after the Red-Line, boarding at station 83. Its route-map is an upside-down-fishhook shape with the same station-numbering scheme (transfer stations, stairwells at 12/24/36/48/72). Its cars are near-identical to the Red Line's but with English (not Russian) graffiti, blue cushions, and tighter aisles.
The party farms mobs from its platforms, then rides it to explore — meeting the loop-trapped conductor Vernon in car five, discovering the porter Pierre's reward room in car ten, and learning the whole line takes three full days to run end to end (stations 11-435). It's the party's primary route for mapping the Tangle and grinding.
Subway cars with English graffiti, blue inward-facing seats, tight pole-cluttered aisles; a fishhook-shaped route-map of numbered stations.
The party's main working rail line for grinding, mapping, and meeting the train's crew.