Downtown Chicago comes pretty close with subway on the lowest level, trucks and commuter rail above that, and cars/pedestrians on the surface. There is also a large network of pedestrian tunnels which is useful in winter (and when coming from aforementioned commuter trains).

And until 2003, there was even an airstrip only 2mi from downtown.

As a somewhat frequent visitor to Chicago, I like the idea of the pedestrian tunnels, but they don't seem like a practical way to get around.

They're basically the separately-operated basements of stores, hotels, and office buildings networked together. Usually one of them between you and your destination will decide to lock the doors.

I've found myself nearly under my destination, encountering a locked door, and having to backtrack and find my way out through a fitness club.

Montreal's Underground City is a pretty great example of how to give public pedestrian connectivity underground in winter. Well worth checking out: https://en.wikipedia.org/wiki/Underground_City,_Montreal

Yea it's also a pretty common sci-fi / cyberpunk fantasy city design dating back to Fritz Lang's Metropolis, influenced by the Art Deco "city of the future" ideas like this floating around at the time.

In NYC at least the lower levels are already present. But the only part where we got the pedestrian level correct so far is the High Line... which has been hugely successful. There's really no reason these types of park-like promenades only have to be built on top of existing disused structures however. More could be built, it's just that people need to understand that there is return on investment for building an elevated pedestrian-only street level.

Except that in most cities including New York it's going to be difficult to build something like that in the absence of an existing structure and right of way. The High Line is also in effectively a post-industrial area which is now highly gentrified but wasn't previously exactly prime real estate. And it's still not really located for day to day pedestrian movement as opposed to recreation.

Totally agree, it would be a political and engineering mess to try and implement on an existing busy street. The conditions were very ripe to experiment with the high line.

Not an easy thing to develop certainly, but relative to the cost and level of engineering effort needed to build new subway tunnels, I'm surprised that adding transportation improvements at street-grade and then bumping the street up one level, isn't something that ever gets considered. We could run effective BRT through Manhattan, or surface light rail to solve the crosstown transportation problem.

Imagine how much easier getting from Queens and Brooklyn to the Lincoln and Holland tunnels would be by car or bus if say 39th St, Canal St moved the pedestrians to an elevated walkway. What if Times Square's tourist bottleneck could be eliminated by raising Times Square?

Anyhow, unlikely to happen probably ever, but fun thought excercise

For an extreme (and somewhat tongue-in-cheek) version of that, see The 5th Element. Everything happens in the sky and street-level becomes this shady area filled with trash and thugs. :)

Or Star Wars' planet Coruscant, or Futurama's New New York.

Or Deus Ex: Human Revolution's Shanghai