ONTARIO, Calif.—The unsolicited proposal from Elon Musk’s tunnel-building venture arrived in January 2020. To the local transportation authority, it felt like finding Willy Wonka’s golden ticket.
Officials had started planning for a street-level rail connection between booming Ontario International Airport and a commuter train station 4 miles away, with an estimated cost north of $1 billion. For just $45 million, Mr. Musk’s Boring Co. offered to instead build an underground tunnel through which travelers could zip back and forth in autonomous electric vehicles.