CHICAGO — The $8 billion in stimulus cash awarded to 13 high-speed rail corridors across the country may seem like a windfall for advocates, but there's a catch: The money isn't enough to finish any of the major projects.

State coffers are dry and federal spending is being cut back, so it's unclear who, if anyone, will pay the rest of the multibillion dollar bill.

Many states have been vague about how they would foot their part of the bill. But experts say most are counting on the federal government to cover at least half of their costs over the next few decades — a hope that may clash with President Barack Obama's recent pledge to curb spending.

Optimists point to the 2011 federal budget Obama proposed Monday that seeks $1 billion more for high-speed trains on top of the $8 billion he announced in stimulus money last week. There's another $2.5 billion tucked away in the 2010 federal appropriations bill that has been approved but not yet allocated.

A proposed $500 billion, six-year federal transportation reauthorization bill includes $50 billion for high-speed rail. But that generous sum was included before Obama began talking about belt tightening.

Even if it came through, that money hardly covers the proposed price tag of the 13 high-speed rail corridors, which are estimated to cost at least $60 billion and possibly more than $100 billion over the next decade or two. Those cost estimates also don't include the hundreds of million of dollars it could cost each year to operate the networks — costs that states typically pick up.

"States have to be very, very careful, and realize that it might be hard for the feds to kick in the money for high-speed rail," said Scott Pattison, executive director of the National Association of State Budget Officers.

It also might be hard for states grappling with budget shortfalls to justify spending more on high-speed rail.