Just to get warmed up, chew on this -- from 8:00AM to 8:59 AM on an average Fall day in 2007 the NYC Subway carried 388,802 passengers into the CBD on 370 trains over 22 tracks. In other words, a train carrying 1,050 people crossed into the CBD every 6 seconds. Breathtaking if you ask me.
Mike Frumin analyzes subway passenger counts. Staggering numbers, but even moreso when you consider (as he does) the number of vehicles (and vehicle-related infrastructure) you'd need without the trains.