A national high-speed rail network would not only support tens of thousands of jobs but also would revitalize struggling downtowns, help save the environment, and cut delays for travelers using highways and airports. President Obama set a goal of providing 80 percent of Americans access to high-speed rail within 25 years. But that vision is yielding to high costs and determined opponents. Virtually none of the projects have gotten off the ground, and the one that has is in trouble.