Behold, the mighty St Louis Lambert International Airport!

It’s a massive complex with four runways, more than 80 gates, a Boeing (formerly McDonnell Douglas) manufacturing complex, and a national guard base. Or at least, it was at its peak. But St Louis, like many mid-size cities in the middle, has seen its shine tarnish, and the airport has suffered. To bastardize an old saying, they built the church for what they thought was a Wednesday, but it turns out it was built for far more than even Easter Sunday needed.
When the terminal complex was first built in the 1950s, it was a modest affair with three concourses. You’ll see the basic original footprint in black below.

Those concourses were renovated and/or rebuilt in the 1970s/80s when TWA and Ozark really focused on growing their operations at the airport. Concourse A on the upper left was extended while Concourse C on the upper right grew even more. And then Concourse D at the bottom was built. That yellow and black outline was the steady state during the heyday of the airport. TWA bought Ozark in 1986, and the airline kept growing St Louis as its main domestic hub.
But it wasn’t just domestic, TWA grew into a global carrier from St Louis with B747s to multiple cities in Europe. The hub was buzzing, and St Louis became the airline’s headquarters in the early 1990s. This was a red airport through and through… until the change.
In the 1990s, another airline started to grow in St Louis: Southwest. By 1998, the East Terminal was opened in St Louis off the end of Concourse D, creating a home for that airlne with its own check-in and parking areas away from the main terminal. The airport was bursting at the seams.
And then, it all fell apart.
TWA had been wobbly during the 1990s, but the airport somehow didn’t see it. It commissioned a third runway, 11/29, to be built at the northwest part of the airport. That didn’t open until 2006 when it was clear it would never be needed.
By then, TWA had been taken over by American (in 2001) and despite an early effort to keep the hub going, it only shrank from that point. The hub officially closed in 2009, the same year the Air National Guard unit left the airport as well. Suddenly, St Louis was a much quieter airport.
As with other mid-size airports, local flying did pick up to backfill the vacuum left by American when it closed up shop. But the airport had far too much terminal real estate, so it started shutting things down. Today, it looks like this:

Concourses B and D are shut down completely, though Southwest did extend its terminal into the end of the old D a little to make sure it had enough gates for its growing operation.
Meanwhile, the airport’s infrastructure is old and in desperate need of updating. Enter the airport’s modernization plan.
So far, STL has focused on fixing some of its issues around the airfield. It has moved its airfield maintenance and is building a new deicing pad on the west side. There are a variety of other fixes that will help to operate like a smaller airport would.
But it’s the terminal complex itself that needs the most work. The airport entrance off I-70 will be moved a little further west, and the current garage will be replaced with a bigger one alongside a new ground transportation center.
The headhouse itself has to stick around. Unlike other cities, St Louis has a deep love for its headhouse because of its four iconic domes:

This will continue to be where passenger processing is done, but they will be completely remaking the inside of the space. The concourses, however, well… nobody has any love for those. So the new plan is to build a single replacement concourse with 62 gates that will look like this:

The new concourse will be directly connected to the headhouse via a single TSA checkpoint, and it will sit on the footprint of the original 1950s-era concourses, simply spreading further west and east.
When this is done sometime next decade, the East Terminal (now called Terminal 2) will close, and it will be a much more compact design, all this for a $3 billion price tag.
I’m not sure how the phasing will be done here, but it shouldn’t be all that hard. With so much unused space, they can presumably knock down the A gates and rebuild that half of the concourse while relocating the airlines temporarily. That’s the easy part at an airport like this one which grew far too big for even its wildest dreams.
