Agreed. The idea of the "pneumatic train" (http://nycsubway.org.s3.amazonaws.com/images/articles/beach-...) has been around for at least 100 years, so why does everyone with high-speed ground transportation go rail?

Existing lines and infrastructure? Perhaps. But we too have existing lines and infrastructure (train stations on those lines).

Or is it perhaps that building and maintaining miles and miles of rails, vs. tubes, is just way, way more cost effective?