Kenton Ngo made an animation showing how many people are entering or exiting Metro stations at each hour across the day.
Green circles show where people enter, and red where they exit. As you’d expect, green circles swell and then shrink at end-of-line and other busy suburban stations in the morning, while even larger red circles appear at the stations at major job centers. In the evening, the pattern reverses.
This is another way of visualizing the Metro station data which WMATA released last year. Matt Johnson used it to compute the busiest stations and the balance between stations. In 2009, Matt diagrammed the flows in each direction.