City officials hope for construction to begin within a year, and to last less than three years. But much remains uncertain. Mr. Emanuel’s administration must still negotiate a contract with the company. The Chicago City Council must give its blessing. Even if the project comes at no cost to taxpayers, it is unclear how extensive the costs would be: Early estimates have reached close to $1 billion.

A project like this appears never to have been completed before. Mr. Musk’s company describes the Chicago project as a taste of the transportation future, a way to reduce congestion and to get around faster by traveling in underground tunnels on a “platform on wheels propelled by multiple electric motors.”

Mr. Musk’s company, the Boring Company, is already working on a test tunnel project in Hawthorne, Calif. It has applied to build another tunnel in Los Angeles and is proposing another between Washington and Baltimore. The company’s plans have run into regulatory questions and skepticism from local officials in California and on the East Coast.

In Chicago, Mr. Musk’s project would be a way for taxpayers to recoup their investment on an unused transit superstation beneath a downtown shopping center that had been envisioned in the early 2000s as a starting point for express trains to O’Hare. That site — which has at various times been described as a “money pit” and a “superstation to nowhere” — reportedly cost more than $250 million, and was eventually mothballed. At O’Hare, a new station would have to be built to accommodate the new service with skates.

For Mr. Emanuel, a Democrat, the new project is in line with his plans for O’Hare’s future. An expected expansion will increase the airport’s gate capacity by 25 percent, city officials say, and nearly $500 million has been promised to modernize the L line to the airport.

The city solicited proposals last year for companies that could build a transportation option delivering passengers to O’Hare in 20 minutes or less, and in November, Mr. Musk said on Twitter that his company was vying for the contract.

If completed as planned, subterranean rides to the airport would cost no more than a trip via taxi or Uber, but probably significantly more than the L train, which takes about 40 minutes to get from the Loop to the airport and offers round trips for $7.50.

Mr. Emanuel, who is up for re-election in early 2019, faces a vast field of challengers, and it is unclear whether they would move forward with the project if elected. They include Garry F. McCarthy, a former Chicago police superintendent; Lori E. Lightfoot, a former Chicago Police Board president; and Paul G. Vallas, a former chief executive of the city’s school system.




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