Almost everything I've learned about walking through urban centers in the wintertime has come north of the border.
That's because Canada's main cities invariably come equipped with networks of underground pedestrian tunnels or other indoor walkways and bridges. In the heart of Montreal, for example, there's an underground storefront to match most street-level storefronts. In the heart of Winnipeg, one can walk several miles -- or kilometers -- without ever experiencing the weather. (You learn this one in a hurry, believe me.)
Our Downtown isn't Canada, of course. Heck, our city's not even as cold as it once was, and we seldom see any significant snowfall. But we do have one pedestrian tunnel, even if it's one that's probably only known to the people who've worked in our two tallest skyscrapers, the U.S. Steel Tower and the BNY Mellon building.
Up for a little tour?
I took a walk after business hours to give you a glimpse, beginning with the gorgeous Mellon Green parklet -- pictured atop this article -- that rests between the buildings, then entering the Steel Plaza subway station that's built under BNY Mellon:

DEJAN KOVACEVIC / DKPS
At which point you proceed as if heading toward the trains ...

DEJAN KOVACEVIC / DKPS
... then down a loooooooooong escalator to access a gorgeous, solidly structured tunnel that connects the two skyscrapers underneath Sixth Avenue:

DEJAN KOVACEVIC / DKPS
It's got murals depicting civic history along the winding path:

DEJAN KOVACEVIC / DKPS
There are even shops, eateries like Au Bon Pain, two mini-cafes and an old-fashioned shoeshine stand, all of which are now open and active again post-COVID:

DEJAN KOVACEVIC / DKPS
And then, once one emerges, it's the basement level of the US Steel Tower, with an escalator rising up to the grand lobby ...

DEJAN KOVACEVIC / DKPS
... which looks out over the US Steel Tower's plaza facing the place where you just came from:

DEJAN KOVACEVIC / DKPS
Neat, right?
And you can do the whole thing in, like, five minutes depending on whether you stop and shop in the tunnel. And it's all open to the public.
Why's it there?
That's easy: In the 1980s, when Downtown's streets were chopped up to pieces to create the subway system, the former Mellon Bank was erecting what's now BNY Mellon. So the two concepts were merged to make the Steel Plaza station, our largest by far, and have it serve the tens of thousands of people working in both skyscrapers rather than just the one.
We're currently at around 65% of our usual workforce in the Golden Triangle, according to the Pittsburgh Downtown Partnership, and that figure keeps rising. Ideally, it never makes it back to 100% because it'll mean we'll have converted many more of the older buildings into residential, something we should've been doing long before COVID made it a must. Our residential occupancy, even with six more such conversions underway, is over 93%, with some places putting people on waiting lists.
Still, skyscrapers like these will always, always be office space. They're designed that way and, more important, they come with amenities like this tunnel that'd never be added to anything residential. The bounceback for them will take a good while longer, per experts -- especially BNY Mellon, which is undergoing an ownership change a few years from now and doesn't seem to know who/what it wants there. But the advantage they both have is in being Grade A in their category.