I once lived in a building between a very busy railway line (mixture of electric and diesel trains, passengers and freight) and an average road (cars and one bu

I once lived in a building between a very busy railway line (mixture of electric and diesel trains, passengers and freight) and an average road (cars and one bus route). The windows facing the railway would get coated in a black grit. Slightly oily particles, just sticky enough to stick to the glass. The windows facing the road would get coated in a dark brown slime. According to the air quality maps of the city, the road is slightly more polluting than the railway, although the railway is carrying 100,000 passengers a day.