Two cities of a certain size that are within the distances that high speed rail makes sense over driving or flying. It makes sense to connect Los Angeles to San Francisco, for instance. However, there isn't that much density in a large part of the country to justify the cost of high speed rail currently.

Hell, a big complaint with California's HSR implementation is that it isn't initially connecting either of the two main cities; those segments are still under design.

So a high speed rail trip cross country isn't going to be viable any time soon, and likely shouldn't be planned for beyond mandating a federal electrification and signaling standard.