As I see it, noise is just as much a pollutant as smog. Smog hangs around and destroys lungs and wildlife. Noise goes away faster, yes, but still provably causes mental damage, as well as destroying wildlife. Any solution needs to consider all pollutants, including noise. If this means using fast, underground bullet trains then so be it. The startup costs would be huge, but the overall benefits might be worth it. Pity that our society does not reward forward thinking, only short term compromise.