> Is there a fundamental reason why the subway must be like that? 1) Dust from brakes, as noted by other comments. 2) Tunnels, and stations in underground spa

> Is there a fundamental reason why the subway must be like that? 1) Dust from brakes, as noted by other comments. 2) Tunnels, and stations in underground spaces, therefor hard to ventilate. > I assume it's not as bad in say Hong Kong? No idea, but air quality in the London underground is very bad indeed. I mean you can sometimes see a visible brown haze looking down the length of the platform. https://airqualitynews.com/2019/11/29/air-pollution-on-london-underground-four-times-worse-than-above-ground/ https://airqualitynews.com/2019/11/29/air-pollution-on-londo...