Quick googlage tells me 5% of the US takes public transit, while it's around 60% for Japan (closer to 90% in cities). I have said from the start being shoved into a moving metal box (recirculated air) with a bunch of strangers is likely the number one cause of the spread (which is why large cities have higher rates than rural towns regardless of mask compliance).
Masks might help, and in Japan they might help a lot, but we can't tell because there are other variables in play that have a much greater impact.
To clarify, using bullshit numbers I made up, if the bog standard rate of infection with normal exposure and zero precautions were 10%, and masks reduced that by half (5%), but mass transit tripled your exposure (15%) you would be worse off wearing a mask in a big city using mass transit then just going about your life in a small town. Doesn't mean those in the big city shouldn't wear masks.