If you stand in the middle of a metro station at its busiest hour, you will see a subway car stopevery 90 seconds. It lets off hundreds of people, and hundreds of people get back on. Peopleleaving and entering the station walk shoulder to shoulder, packed like sardines1. On thesubway car itself, there may be little standing room left, and sometimes nothing to hold on to.When the car goes around a corner quickly, people just lean against each other to keep fromfalling.