First appears in: The Metal Wood
The underroad is a secret network of high-speed tracks and repurposed infrastructure built by the gearheads. The prose describes the experience of navigating it: "Bars of white fluorescent light whipped by her as she drove, speeding through the Under racetrack that the gearhead squad had built... Cement whipped by at sixty miles an hour, dipping and curving and barely lit, headlights racing to stay ahead of them as they flew". It is composed of "water pipes and sewers [that] can be repurposed for driving," and in some sections, the road even "rotated ninety degrees, turning into a wall" to allow for vertical travel.
Created over years of work, the underroad allows the gearheads to "transport goods and people quickly" and even bypass government security to enter "Planner Central". The network includes the "Centrifuge," a massive vertical pipe where centrifugal force pins cars to the walls as they spin upward. It is the setting for the gearheads' high-stakes metalwood heist and their subsequent desperate flight from Planner fallcars, which resulted in the near-total destruction of the squad.