PH,
I generally agree. If it's done correctly (high-speed between the big cities with few stops plus regional trains to smaller towns and suburban trains to the suburbs) like it is in Germany, a national rail system is great. I love the Deutsche Bahn - nothing like stretching out and watching the countryside fly by while you're sipping some fine German beer. However, most of the US is just too spread out to do that.
Scale does matter. However, you don't quite have to go fully national. For example, I've always heard that the Amtrak lines that run through the Northeast (including the one "sorta-high speed" line) actually make good money and help subsidize the rest of the system, which is a consistent loser.
What I would do is get rid of Amtrak as a national system. Let the states in the northeast form a multistate rail authority and operate the profitable lines. Instead of taking those lines' profit to fund a bunch of mostly empty trains throughout the country, let them use that money to lower fares or improve/expand their current service.
If other states want to experiment with their own lines or form other multistate authorities where they think they can operate efficiently, let them try it. If California wants a high-speed line from San Francisco to Los Angeles or if Texas wants a Dallas to Houston line, let them try to make it work. However, they shouldn't be getting any federal money or ripping off the northeastern lines that already work.