[Symbiote]

There are already buildings built over railways (as well as many railways tunnelled under things). But they're much safer (hardly any accidents, trained people who know evacuation routes etc) and the noise and smell isn't as bad as it would be for a road.

The examples I think I've seen are hard to verify on Google Maps, it's a brown-grey mush, and in any case they're all odd buildings rather than a 'tunnel' created out of buildings.