They do not always wear masks. They sometimes did in the past. Now they all have masks. In the past, you might see one or two people in a train car wearing masks, mainly for their own comfort, as I explained above. Now, everyone wears one. I am sure there is a tremendous difference in the infection rate for colds and influenza between the situation where one or two people wear masks, and everyone wears one. Previous years are not comparable.
I spent a few weeks in Japan last year. About 30% of people on subway trains wore masks. On city streets, about 15%. On intercity trains and rural buses, and in rural towns and villages, very few. In restaurants and shops, virtually none. On the hiking trails, none at all.
So far as I was aware, I caught no viruses; despite taking many subway journeys at busy times when whatever the opposite is of social distancing (?social cramming?) is practiced