On Wednesday, the city of Chicago opened a Request for Qualifications (RFQ) for an express train that would take passengers from the city’s O’Hare airport to downtown. The system would have to be completely privately funded—Chicago says no taxpayer money would be used for it.

Elon Musk’s Boring Company—a tunneling company that the SpaceX and Tesla CEO started last year—will respond to the request. Musk hopes to get to the second round when bidding will take place. On Wednesday evening, he tweeted that his company “will compete to fund, build & operate a high-speed Loop connecting Chicago O’Hare Airport to downtown.”

Musk’s reference to a “Loop” is explained more clearly on The Boring Company’s FAQ page: “Loop is a high-speed underground public transportation system in which passengers are transported on autonomous electric skates traveling at 125-150 miles per hour. Electric skates will carry between 8 and 16 passengers (mass transit), or a single passenger vehicle.” Unlike Musk’s idea for a Hyperloop, a Loop won’t draw a vacuum. “For shorter routes, there is no technical need to eliminate air friction,” The Boring Company states.

The company also clarifies the concept of an “electric skate:” that is “a platform on wheels propelled by multiple electric motors.” The platform would operate autonomously without a rail or rails to which the skate would connect. The skate would operate in the tunnel’s main artery, and it would enter and exit from side tunnels. With this system, The Boring Company says, the skate’s average speed would theoretically be able to operate close to maximum speed.

Mayor Rahm Emanuel has called for an express transit link between the city’s main airport and downtown to be in planning within the next three years. According to the Chicago Tribune, the city is accepting project proposals that are above ground, too—a tunnel is not strictly necessary. A source told the paper that such a project would likely cost between $1 billion and $3 billion.

Wednesday’s RFQ specified that any service that the city would consider should offer travel times of 20 minutes or less between the two hubs, with departures every 15 minutes. Any “premium” fares would have to cost less than any ride-hailing service.

The mayor’s office offered three potential routes but didn’t limit interested companies to those routes. The Tribune says those routes include one that follows “the current Blue Line route along the Kennedy Expressway,” another that follows “the Metra North Central route,” and a third “could be a combination of freight rail rights of way that would go west from downtown south of the Eisenhower Expressway out to Forest Park and then turn north toward O’Hare.”

The new plan echoes a failed idea from a decade ago, under Mayor Richard Daley, when the city and its transportation authority “put more than $250 million into a hole in the ground before work stopped in 2008.”

Back in June, the Chicago Tribune reported that Mayor Rahm Emanuel had had a discussion with Musk about building an underground tunnel in the city, but those discussions were described as “preliminary.”