It could be easier to work on the problem by talking about it differently.
Under current law and definitions, trains are long distance or state-supported because they are not long-enuff distance. (Not getting into local commuter systems.)
When Amtrak was formed, the system included the long distance routes. of course. It also included a bunch of shorter corridor trains that mostly overlay some LD routes. The NYC-Buffalo Empire Service, for example, and the St Louis-CHI Lincoln trains, and a few others.
As years went by, various states made deals with Amtrak to start or expand corridor service, and at some point someone noticed, and began to agitate, that some corridor trains got a lot of federal Amtrak money and others didn't much or any help at all.
The PRIIA solution equalized things by treating all, or almost all (not the Palmetto. for example), the non-LD trains the same, by shifting 85% of the costs to the states, while Amtrak covers 15% of costs recognizing that the corridor are some part of the national system.
The either/or line was set somewhat arbitrarily at 750 miles.
With the passage of time, it's clear that the 750-mile limit has discouraged states from adding corridor service where they have to pay 85%, especially where states (like New Hampshire, Alabama, Mississippi, and others) will not agree to partner their fair share of the costs.
Most of us see the corridor trains as part of the national system, to some degree or another. The Cascades trains Eugene-Portland-Seattle-Vancouver, BC seem as much a part of Amtrak as the Coast Starlight on most of the same tracks.
But the degree any given corridor is actually part of the national system may differ considerably. The 750-mile breakpoint is too crude a measure.
Looking at the actual miles involved, it's clear that NY-Albany are NY State trains. Extend them to Buffalo, well, hmmn. Take them to Montreal or Toronto and why the hell are NY taxpayers covering 85% of those costs? Chicago to Milwaukee, yeah, that seems like something for Illinois and Wisconsin to handle. But CHI-St Paul with 418 miles? Or CHI-St Paul-Fargo with 662? Maybe CHI-St Paul isn't quite the same level of 'national system' as the Empire Builder is, but it seems at least halfway national. And CHI-Fargo -- a four-state corridor over 650 miles, that's national system.
It's the 750-mile limit that's most in need of changing. Leave NY-Albany and CHI-Milwaukee for the states. Set the cut-off limit for 15% federal cost-sharing at 200 miles. Then have the states pay 50% of routes between 201 and 400 miles, with the feds, that is, Amtrak, paying the other 50%. Make that 400-mile figure be the new maximum requiring state support. Any route of 400 miles or more will be part of the national system like any other LD train.
If we could get the 750-mile limit cut almost in half, and federal money to help support middle-distance trains, we'd see a clamor for more trains to expand the Amtrak system.