Tunnels. I'm not kidding. Much cheaper tunnels would completely restructure real estate, so there are trillons of dollars at stake. Yeah you're not the perfect team for that, but you can hire the perfect team for that.

Examples:

- at 200kph, a 50km commute takes 15 minutes. Draw a 50km circle around San Francisco (all the empty land near Walnut Creek or in Marin County now "local" to downtown)

- at 1000kph, transporation between cities becomes better and faster than private jets, for everyone (no need to drive to the airport)

Edit: Tunnels are still the most underrated big idea out there. Of course those numbers come from Elon. "Without tunnels, we'll be in traffic hell forever" - Elon Musk.