Greater Greater Washington maps rider flow for the DC Metro. As you might guess, the thicker the path, the greater the estimated number of riders in that given area.
As the author notes, the data collection process was an unscientific one, so it should be taken with a few grains of salt, but this makes me wonder. These types of subway maps seem to be getting fairly common, in both the static and interactive/animated variety - but the visualization always seems to come from estimates.
Have any metro systems released their full data? I am sure there are tons of data logs sitting somewhere, growing every time someone swipes their metro card or drops in a subway coin. And more importantly, are metro systems using these types of visualizations to figure out how to distribute trains at different times per day? Do they use something better?