[mozumder]

This was possible because cities weren't as developed as they are now. Property was cheap back then.

These days, the major expense of building a mass transit system is just buying property. And in cities, these properties are going to be already developed.

Right now, there are individual buildings in US cities that cost over $1billion, so grade-separated ground rail is not going to happen for cheap. That only leaves tunneling, which also isn't cheap.