By Day, By Night: Manhattan's Population

As explained by Frumination:

-- 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.


via Gizmodo (you can also enlarge the map here).