Here in Tokyo the rail system sees an average of 40 million rides per day. That is 8 times as many as New York City, yet the coronavirus has spread very slowly in Tokyo. Single neighborhoods in New York have seen more deaths than all of Tokyo combined.
What is overlooked seems to be the health of those infected. Healthy people seem to suffer few or no symptoms, whereas unhealthy people become seriously ill or die. The carrier Roosevelt is a good example, the sailors on the ship were healthy, and the virus had little effect on them, 60% of those who had the virus showed no symptoms at all, while most of the rest had only minor symptoms. Those who are most likely to be seriously affected by the disease are the obese, those suffering from diabetes, and smokers. Obesity and diabetes are said to be at epidemic levels in America, while in Japan they are quite uncommon. You won't see many overweight people on a Tokyo subway. Your average Japanese commuter walks a hell of a lot, and climbs a lot of stairs. Even a modern city like Tokyo has tens of thousands of multi-story buildings which do not have elevators or escalators. On my old commute, I used to walk about 15,000 steps per day, and I would climb or descend at least a dozen flights of stairs. There were the stairs at the station, the stairs at my building (my office was on the 3rd floor), at restaurants, and shops.