https://www.wired.com/story/subway-air-health/?utm_source=facebook&utm_medium=social&utm_brand=wired&utm_social-type=owned

Found this kind of interesting because for one I want to expand rail, and second of all cities like NYC actually have very low average concentrations of fine particles on thr street level. I didn't know they were so high in the subways, or that fine metal particles even hung in the air enough to be measurably high. I'd have thought they'd quickly settle on the ground, and maybe that is actually the case but trains come frequently enough to kick it all back up.

It seems likely that the fine particles emitted by car tires and brakes are still probably more harmful to health, mostly because those are the "micro-plastics" that get lodged in your body and are difficult for it to remove. Whereas as the article says, some of the metal that gets thrown up by trains is soluble iron, so the body can basically absorb it and treat it like iron concentrations in water supplies. There are biological means of effectively regulating some of the particles, whereas micro-plastics can just remain in your body leeching carcinogens and irritating tissue.

Not sure what should be done that is cost effective. It mentions some stuff like platform screen doors and vacuums.