My column in Forbes is up for the week, and discusses the failure of light rail. In particular, it focuses on Phoenix light rail, which has been hailed by the intelligentsia as a stirring success. Which it is ... if you are willing to completely ignore its costs. Saying that Phoenix light rail represents an example to be emulated is roughly equivalent to saying that an Aston Martin makes a sensible middle class family car.
One reason Phoenix is a particularly bad candidate for a light rail line is that our population is so dispersed, and there are not any obvious commuting routes. Our downtown is a destination for very few, but even here the commutes, as shown on this distribution map, are from all over, hardly very good fodder for rail (the downtown is near the "phoenix" label). More importantly, people work all over, so taking a suburban zip code, look at where people are commuting to from suburban 85032. Again, all over. Notice how few are going downtown (where the light rail line is -- downtown is toward the south about where the "phoenix" map label is). In other words, people in Phoenix are driving from all over to all over.
Update: Now here is my idea of rail running in the streets, via Shorpy