The population distribution in most of the US is simply not geared toward passenger rail except possibly at the local level
That's not really true. It rarely makes sense to extend light-rail systems beyond the densely packed urban centers, but you're ignoring the old heavy traffic. The layout of our towns, highways, etc are all heavily determined by the paths that the railroads took 150-75 years ago. This hasn't changed, as many of our Interstates were built along similar pathways.
Now, Amtrak may suck, but it's not like there's good competition available. Driving takes every bit as long and already costs far more, and our piss-poor airlines with worse food than a Flying J: Don't even get me started on the Fly America Act and even greater sins our government commits in their favor.
If we had new rail-systems and new stations (with ZipCar and other car rental companies etc. colocated thereupon), they might very well be able to perform profitably. Let foreigners run 'em, too, so that the food doesn't taste worse than the truck stop food you'd get when driving (which is still better than the nothing-to-ramen spectrum on American air carriers), and this may very well be worthwhile. If speedy rail systems can be built that are fast enough and substantially more environmentally sound, we might even consider taxing competing air routes to subsidize them in an effort to meet soon-to-be-adopted CO2 emissions goals. Of course you may wish to hold off until after opening them up to all comers to knock the price down an equivalent amount.
Regardless, I'd assert that there is a market for a competently run Amtrak with maglevs et al or, better yet, multiple competing private firms. We just don't see it right now because the Amtrak service is (marginally) worse than the (insanely bad) domestic airlines. If we can restore service to all the cities over the million-person mark, I think they'd do just fine.
They just can't compete as long as:
1: They're as slow as a car
2: They serve worse food than truck stops (like the airlines)
3: They fail to advertise and compete aggressively due to lack of real market pressure
4: They fail to service many large cities
Still, that's half the point of the above. Look beyond light rail - The car manufacturers can make a lot of money regearing to deal with the above issues. If they're going to be bailed out with taxpayer money anyway, perhaps we should lead them in this cheaper and more fuel-efficient direction.