Propagation on subways is a possibility, but then again, Japan has oodles of above ground railways and underground subways.

However, the majority of propagators are those from 16 ~ 29 years of age. And of those, the ones 22 ~ 29 are well known for frequenting drinking hot spots which have been discovered to be spreader sites... (i.e. not the trains or subways).

But in NYC, on a subway with some no mask individuals... I could imagine that would be one possible spreader situation.

FWIW

Subways in Japan are as clean as a private home.  Subways in NYC were filthy and dripping with mold.  They have been steam cleaned since then and are considered to be much safer.