Mega-regions, if they are to function as integrated economic units, require better, more effective, and faster ways move goods, people, and ideas. High-speed rail accomplishes that, and it also provides a framework for future in-fill development along its corridors. Just as development filled-in along the early street-car lines and the post-war highways, high-speed rail will encourage denser, more compact, and concentrated development with growth filling in along its routes over time.
I’ll just add that we’re evolving a network economy where modular diy (or bootstrap) business development can take root, and I suspect the future will depend on our ability to connect more than it will depend on our ability to grow. We have technical infrastructure to support connection, light rail could be part of the physical infrastructure. (Thanks to Tim O’Reilly and Steven Johnson for pointing me at this piece.)