The biggest problem is that the cost of acquiring land and building a system from the ground up makes almost any rail transit system cost-prohibitive. Add in the limited passenger capacity even in the most heavily-utilized systems due to signal system specifications, safety requirements, minimum train spacing, etc.
I'm kind of surprised that rail transit is getting a lot of attention at all these days. Over the last couple of years a lot of transit agencies have come to recognize that a well-operated bus system -- including bus rapid transit (BRT) routes that could be as ambitious as a fully grade-separated or barrier-separated busway alignment -- is far more cost-effective when it comes to moving large numbers of people along defined transportation corridors.
The problem is that while bus service may be more efficient, most people say if I’m going to take a bus, I might as well drive.
And in Most inner-cities, unionized bus drivers make more than most AIRLINE PILOTS, with incredibly huge retirement packages.
What do you think of the potential of raised-rail PRT systems? I know there's a regulatory issue with how closely the cars can run together, but if they can get around that, does it make such a system viable?