I don't believe we'd need subterranean travel for the entire way to be useful.
Planning for long-distance travel in automobiles will likely require a variety of solutions. Tunnels make sense getting into and out of densely populated areas, and onto long-distance highways. Reserving lanes for autonomous vehicles that communicate with the grid and understand destination information and at some point, can influence the average rate of travel would also be helpful. Combining these two simple solutions (tunnels and exclusive use highway lanes) can drastically reduce travel times.
High-speed trains, if ever implemented in the US, work nicely together with ride-hailing services, like Uber and Lyft. Getting down to DC from NYC works very well on the Acela, although it is not cheap - it can compete effectively with air travel.