The Northeast real estate market bout to become San Francisco prices if such proposal of a mega region and train corridor comes to reality.
Your point is valid, but depends on land use and land use restrictions.
San Francisco/Bay is constricted by mountains, the Bay itself, National Parks, land use restrictions that stop growth into San Joaquin Valley, and even in the case of San Jose, they can't build very tall buildings there because the airport is to close to downtown area.
Northeast Corridor still has lots of suburbs and inner cities that are "underdeveloped"....and then if you build something like a Hyperloop, places as far away as Richmond, VA become good value locations to build a home....because at those speeds of 300 miles per hour or whatever ridiculous speed your commute is still reasonable to get to jobs in DC or Baltimore area or beyond....and vice versa.