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I believe at least subways tend to have lousy air: https://engineering.nyu.edu/news/subway-air-pollution-disproportionately-impacts-new-york-citys-minority-and

• masklinn 29d ago
  I was thinking open-air yes, tunnels do indeed have major issue. With pollutant concentrations, road tunnels are the same (https://www.sciencedirect.com/science/article/abs/pii/S1361920920306659 https://www.sciencedirect.com/science/article/abs/pii/S13619...). For all that I did not care for them originally this is probably something full-height platform screen doors help with, assuming the trains themselves are also filtered that would keep the pollutants clear of drivers and users, although it does not exactly resolve the question of managing their presence in the tunnels themselves (and track / tunnel maintenance personnel should probably already be wearing respirators if not full breathing apparatus), and I could see things get pretty dire in case of breakdowns.
  • rsynnott 29d ago
    Impossible to retrofit, of course, but what helps more are big tunnels. Modern ~9m diameter single tunnel systems are a lot better for this than old 3.5m diameter double tunnel systems.