My theory is that traveling in subways means the folks who catch it do so with a high viral load.
“Under such circumstances the virus receives a massive jump start, leading to a massive innate immune response, which will struggle to control the virus to allow time for acquired immunity to kick-in while at the same time leading to considerable inflammation and a cytokine storm.
“For most of us, it’s hard to see how we could receive such a high dose; it’s going to be a rare event. In the COVID-19 clinic, the purpose of PPE is to prevent such large exposures leading to high dose infection.”
https://www.sciencemediacentre.org/expert-reaction-to-questions-about-covid-19-and-viral-load/
Those of us in more spread out areas, where almost no one ever spends time in a metal tube with a lot of strangers, may catch it in some repeated tiny doses, giving our bodies more time to ramp up a defense.
I also suspect general filth in a city raises the problem, which may be why Tokyo has done better than NYC.
The subways are the filthiest part of the city. The platforms are filthier even than the trains. The entire subway system needs to be completely and thoroughly rebuilt so it can be kept clean. Then NYC needs to spend the money to keep it clean.
The new Second Avenue extension of the subway is clean and well lit and somehow manages to stay that way. Everywhere else the filth hits you when you take that first step down from the street. Must be the world's most prolific incubator for germs and viruses (virii?).
One of these days neglect of the subway is going to kill an entire city and region. You can bet your bottom dollar the Wuhan virus is only the first of many more to come.