Elon Musk's Boring Co. has won a bid to build a high-speed link between downtown Chicago and the city's main airport.

The proposal makes little sense.

One of its flaws is its low capacity — as described, it can hold a lot fewer passengers than a New York City subway line.

The Chicago Tribune reports that Mayor Rahm Emanuel has selected a bid from Elon Musk's Boring Co. to build a high-speed link between downtown Chicago and O'Hare Airport. Autonomous vehicles are supposed to make the trip in just 12 minutes, at over 100 mph.

It sounds cool, but the proposal as described makes very little sense.

The Tribune says the project "would be able to handle nearly 2,000 passengers per hour in each direction." This is not a high capacity. You can fit 2,000 passengers on a single, crowded New York City subway train — and a subway line can handle 24 of those trains per hour. Even assuming a more comfortable subway load of 1,000 passengers per train, this system would have 1/12 the capacity of a subway line.

The low capacity would stem from the small size of the vehicles, which would carry just 16 passengers each. What is the point of using such small vehicles in a system that connects just two stations?

This all assumes Boring can deliver on its promise of much faster and cheaper tunnel construction, which will rely on technologies the company has not yet shown can work. I'm glad Musk is trying to innovate here — underground rail projects in the US are far more expensive than they should be, and the space is ripe for innovation.

But I'd like to see him show that he can actually build faster and cheaper at a demonstration site before a major city like Chicago goes into business with him.

Plus, one of the ways the Tribune says he'd save money is concerning: His tunnels would only be 14 feet in diameter, about half the size of a typical subway tunnel. A smaller tunnel means smaller vehicles and lower passenger capacity, which can mean it's a false economy — you're spending less because you get less tunnel when construction is finished.

The best argument for this project is that Musk says the Boring Co. will take on the entire construction cost, which he thinks will be about only $1 billion. So long as that remains true, sure, let him have at it, and see if he can actually build the thing. (I wouldn't want to be one of his investors, though.)

But private megaprojects like this have a way of turning into public ones, when costs exceed projections and private operators say they have no way to finish construction without public support. If Chicago goes down this road, it needs to be prepared to say "no" if Musk comes back in a few years and asks for money.