I know we all want fast trains, but if we're going through major cities we likely won't get that fast (unless we do like the Shinkansen and build the entire train on a 3-story platform).
If we're willing to spend $50 billion, let's spend that on some slower trains that connect cities anyways. Maybe you don't do LA-SF, but giving higher inter-connectivity is good.
What happens in Japan is that train stations get built, and then the surrounding area is simultaneously developped with stores and other developments. So you build a train station in the middle of nowhere, but then build an IKEA + a mall. Suddenly people have a reason to take the train! And it helps the local economies too.