The NYC subway seems to have a pretty unique local microclimate though. I don't know if it's just the lack of regenerative braking or if the close proximity to the iconic steam heating network is also playing a role in this, but compared to most other subway networks, the MTA feels like a living history museum - even if you close your eyes, block your ears and only take in the air. The influence of air heat and moisture on virus spread is quite complicated (evaporation ends some droplet classes, condensation ends others) so it's quite possible that some subway systems are much worse spreaders than others.