What real estate? The railway companies got land grants because nobody (well, except the Native Americans...) owned the land they built on. The Boring Company is going to be drilling under already owned land no matter where it goes...
They could option or purchase land, and decide the route of the tunnel based on what they're able to obtain. That puts them in a position to capture value. It could also increase regulatory competition among localities.
I thought they didn't even need to own the land to build tunnels as long as they do it at the right depth?
I think the parent was saying that they would buy parcels of land where they planned to build stations, and then profit when that land appreciated due to the presence of the station.
The term they're using in Australia when politicians waffle on about an East Coast high-speed rail system that's never going to happen is "value capture".