Every major and many minor cities in the world have built them. Look at Tokyo. They built it on solid rock in a major fracture zone of the world because they needed to. Why is it the biggest city in the world? Because they have a multiplicity of transportation systems that are independent of each other. Light rail, which is necessary is still tied in with the surface auto system. If there is a major problem with the auto system it will effect the light rail system as well. There is one major reason for having a multiplicity of rails and that is that the center of the city becomes too expensive to build parking for cars. Y'all are part of the way there by wanting to make Phoenix a Walkable Urban City, but to fully expand you have to have rail under ground in the center of it.