On a recent Monday afternoon at Grand Central Station, strings of subway cars clatter up to the 4/5/6 platform. As trains arrive every four minutes or so, doors open and shut quickly, rarely parting longer than the 10 seconds they’re supposed to. They stick to schedule.
But as afternoon melts into evening, larger numbers of passengers surge onto the platform. “Dwell times” brim over their scheduled mark, as doors linger open and conductors bark at riders to press toward the center of the car. As trains hang longer than they’re supposed to at successive stations, running times stretch further and further past what’s planned.