[theedwood]

> I wouldn't say "easy". It's possible. Here in Seattle, we're building a new 2-mile tunnel (for cars). It was originally scheduled to open 3 years ago, and will finally open in 2019. It was planned to take 29 months, but took 67.

To add to this, some areas are made of materials that are simply difficult to bore through. Atlanta, for example, sits on top of a bunch of granite which at least partially contributes to the difficulties of underground rail expansion.