Last week, Elon Musk made a surprising announcement: The first segment of the tunnel that he’s digging below Los Angeles to supposedly solve the city’s traffic problems was already complete. Musk seems so confident in his idea that he’s now proposing a solution for Chicago’s proposed high-speed rail line to O’Hare—and it’s sounding less like a roller coaster for cars and more like a really fast, really efficient subway.

The Boring Company’s LA tunnel is “a few hundred feet long” according to Musk, although we don’t know where exactly it goes—he hasn’t released a map of the tunnel, but here’s a decent guess from UCLA transportation policy expert Juan Matute—but the typically enigmatic Musk tweet was accompanied by several videos that prove the project is moving forward quickly. Tunneling only began during the last week in January, according to Musk, who admitted at the time, “We have no idea what we’re doing.”

On May 12, Musk posted photos of the tunnel being dug and a video of the LA test track for one of the “sleds” which would carry cars at speeds of 125 mph through the tunnel.

Boring machine segments have been lowered into the starter tunnel. Going through final assembly. Will be a few hundred feet long when complete. A post shared by Elon Musk (@elonmusk) on May 12, 2017 at 5:39am PDT

[Warning, this may cause motion sickness or seizures] This is a test run of our electric sled that would transport cars at 125 mph (200 km/h) through the tunnels, automatically switching from one tunnel to the next. Would mean Westwood to LAX in 5 mins. A post shared by Elon Musk (@elonmusk) on May 12, 2017 at 5:48am PDT

The following week he noted that the electric sled can transport “automobiles, goods, and/or people,” and on June 8, The Boring Company released images of a new “pod” that looks more like a public transit solution.

The two new videos show both the tunnel itself and a prototype of the “car/pod elevator” that would transport vehicles underground and onto the sled. (Musk also tweeted repeatedly last week about his love for floors, so take these tweets as you will.)

Across the road and below the ground pic.twitter.com/E2DG8EzaXg — Elon Musk (@elonmusk) June 30, 2017

Just installed steel skeleton of the car/pod elevator. Should be operating next week. pic.twitter.com/DIZW7zuWaA — Elon Musk (@elonmusk) June 30, 2017

Earlier last week, Musk met with LA Mayor Eric Garcetti for “promising conversations” about how to proceed with his tunneling network without going afoul of city regulations. This tweet seemed to infer that he had secured the necessary permits to tunnel off SpaceX’s property (which is actually in the adjacent city of Hawthorne) and continue beneath Los Angeles.

Promising conversations with @MayorOfLA regarding tunnel network that would carry cars, bikes & pedestrians. Permits harder than technology. https://t.co/0dxrXBOOWy — Elon Musk (@elonmusk) June 18, 2017

But it was Musk’s curious suggestion that the tunnels would “carry cars, bikes & pedestrians” which seemed to pique the interest of transportation advocates, who were concerned that a new tunnel network for cars will just create more above-ground congestion. After all, those vehicles will still have to board this car elevator, single-file, just like they have to merge onto the crowded 405 Freeway that Musk hopes to avoid.

From the look of Musk’s new concepts, it appears that walkers and cyclists will be able to access these communal pods to travel below the city—making the concept much more like a subway than an underground freeway. The Chicago proposal certainly makes it seem more like Musk wants to build a cheaper public transit solution to help people—not necessarily their vehicles—get to the airport fast.

As I’ve said before, if Musk can really upend the tunneling industry by advancing that technology, that could prove immensely beneficial to public transit projects across the U.S. American cities are plagued by high transportation construction costs which have led to serious maintenance issues. As Yonah Freemark notes at The Transport Politic, these challenges have left U.S. transit in the dust while other countries rapidly expand their options for getting around. A narrower, cheaper tunnel outfitted for some kind of rapid-transit vehicles smaller than trains could help some U.S. cities leapfrog their transit projects forward.

It’s too early to tell at the moment, and there are a lot of details to be worked out. But Musk may actually be bringing a viable public transportation solution to the table—not just an underground conveyor belt for cars.