The US is considering the construction of high speed rail corridors similar to those in place in France, Germany, Japan, Italy and, most recently, China, whose fastest current route (Guangzhou-Wuhan) covers 700 miles in about 3 hours. What can we learn? First, only France runs a profitable (in the sense of social welfare) system at present, the economic impacts of HSR are fairly limited (though difficult to measure), the environmental impact of HSR is limited (it produced similar CO2 to cars), and HSR optimally connects regions at distances between 100 to 500 miles that will see over 8 million trips per year. Also, cities with poor internal public transit, or whose rail stations are outside the city center, see less benefit from HSR. Given that around 1/3 to 1/2 of air travel is replaced by HSR after it is introduced (as well as limited amounts of car travel), California and Washington DC to Boston via NY would seem to be viable candidates for profitable high speed rail.