Even within the current framework, it shouldn't be hard to acquire an underground easement, by market or eminent domain. Shouldn't be very expensive, either. A

Even within the current framework, it shouldn't be hard to acquire an underground easement, by market or eminent domain. Shouldn't be very expensive, either. A deep underground tunnel means basically no diminution of property values. Subway and water tunnel projects tend to pay very small token amounts (a few hundred dollars) for similar underground takings. Not quite as streamlined as a "free groundspace" regime, but not a big obstacle.