One of the busiest buses in New York City, the Bx12, starts its route at one end of the A train, in Inwood at the very top of Manhattan, and runs across to Co-op City, in the Bronx—the largest housing coöperative in the world. In between, it crosses a lot of places people might want to get on: the 1 train, the 4, the D, the 2, and the 5; the tip of the Bronx Zoo; the bottom of the Botanical Garden; Fordham University; the Metro-North railroad (Hudson Line); and the Bruckner Expressway, an enormous highway designed by Robert Moses, which cuts large swaths of the Bronx off from the water.

The Bx12 is almost always full. On a recent weekday afternoon, large crowds waited at each stop, and people pounded on the back doors when they couldn’t squeeze on. There is no cross-town subway in the Bro

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