As in Manhattan, the discretization of the space imposed form on how it had developed, with each link of the chain—each city block—acquiring its own skyline and identity. Groups of consecutive blocks had long since coalesced into neighborhoods. Each block was, in effect, a fully independent space vehicle with its own system for keeping the air from leaking out. But each was connected to its two neighbors by a bundle of passageways routed through its foundation slab, which made it possible to move easily from one to the next in the same way that the Londoners of Old Earth had used underground passages—"subways" in the London sense of that word—to cut beneath crowded intersections. Some of the subways were sized for human pedestrians. Four of them carried trains: locals and express service running both directions around the full circuit of the Great Chain. Still others were reserved for robotic vehicles programmed to carry cargo. Beyond that was a wide range of smaller conduits carrying air, water, power, and information. All of them went by the name of subways—this was a conflation of the old London and New York senses of the word. At each end of each block was a system of airlocks; these would seal themselves off in the event that a block were to depressurize. People ran marathons through them—four consecutive marathons made up about one circuit around the entire Chain.