Part of the reason may be that you don’t necessarily see a real cross-section of the population on the subway, either. While most New Yorkers, even the wealthy, commute on it in the morning, many prefer cabs or car services late at night, or stick within walking distance of their homes on weekends, skewing the demographic you see riding the system. As is the case on many subways, different lines carry different types of people — the F, for example, is known for being filled with publishing industry types who commute from brownstone Brooklyn to Manhattan. It also doesn’t touch on many minority neighborhoods, which means it has a very different racial balance from other lines. There are even different proportions of tourists on the subway than on the streets — below ground, they tend to skew European.