Of course American cities aren't built to support it, with their own present state of extremely automotive, exurban decentralization and highly distributed typography. It's a catch 22 that can only be solved by rather preemptive "build it and they will come" style investment, the goal being to slowly influence organic behaviour patterns over time. In the best possible case, people will start moving closer and closer to rail lines, and density of coverage will continue to usefully increase. In some cities that have developed entirely along the automotive paradigm, particularly New South developments like Atlanta (where I live), that may not be possible without a very radical restructuring, given their extremely hostile layout to anything but the car.

Nevertheless, if you give any credence whatsoever to a hydrocarbon-constrained future (and there are plenty of compelling reasons to do so), electric high-speed rail is the only option that makes sense.