From the NYT:
High-speed rail systems look very appealing, whizzing people to their destinations with great comfort and convenience, while generating minimal environmental impact. However, building such a system from scratch is exceedingly expensive, and recouping an appreciable part of the cost through fares is only possible when the traffic density on the route is very high. In other words, high-speed trains must be full and frequent in order for the system to have a chance of making economic sense.

For the full story, please visit http://www.nytimes.com/roomfordebate/2010/10/13/will-we-ever-have-high-s....