"The upfront capital is not there on the public side," said Art Guzzetti, vice president for policy at the American Public Transit Association.

California -- which is seeking to build the nation's first high-speed rail -- is still banking on the federal government covering roughly 40% of the tab. The state is on the hook for 20% -- all coming from $9 billion in bond borrowing that voters authorized in 2008. So far, $258.4 million of the bonds have been sold, according to the Treasurer's office.

For the fiscal years 1997-2008 and through fiscal 2009-10, the rail authority spent $252.8 million on planning, administration and outreach, according to its business plan published in December. The authority has a $221.1 million budget this year, including federal money and $143.6 million from high-speed rail bond funds, according to the Legislative Analyst's Office.

Under the terms of the bond ballot measure, up to $900 million of the bonds can be spent on planning. But the remaining money can't be spent unless a plan is in place to fully finance the targeted rail segment -- and at least 50% of each segment must be paid for with non-state money.

"There is a risk" of spending the planning money without ever building the rail, said David Crane, a rail board member and Gov. Arnold Schwarzenegger's economic adviser. "But it's a governable risk and there's enormous upside associated with taking that risk."

Among the biggest uncertainties is the federal money, and that has investors worried.

Rail officials so far have secured only $2.25 billion of the targeted $17 billion to $19 billion from Washington. Unless there is a dedicated funding source, future appropriations "will have to compete annually with other transportation and nontransportation expenditures, such as national defense and health care," noted a recent report last month by the U.S. Government Accountability Office. The report looked at high-speed rail prospects nationwide and cited the uncertain federal commitment as a concern for private investors.

The California plan also calls for local investments of up to $5 billion, none of which has been committed.

To lure private money, the rail authority says in its business plan that it expects to provide a revenue guarantee to investors "in the event that system revenues are significantly lower than forecasted."

However, the voter-approved bond measure bars the state from providing an operating subsidy -- and the business plan "does not explain how the guarantee could be structured so as to not violate the law," the nonpartisan Legislative Analyst's Office said in a January report.

The rail authority responded in a February memo that it doesn't plan to subsidize operations and that current projections show that the rail would generate a profit. Still, the authority leaves open the possibility of providing investors short-term financial help should the rail not meet revenue benchmarks in the early years.

Revenues will depend on how many people ride the rail -- and the authority's projections have come under fire as of late. In a report issued last month, the Institute of Transportation Studies at the University of California at Berkeley criticized the authority's forecasting as flawed. For instance, researchers said the model gave a "distorted view of the tastes of the average California traveler."

"As such, it is not possible to predict whether the proposed high-speed rail system will experience healthy profits or severe revenue shortfalls," Samer Madanat, director of ITS Berkeley and UC Berkeley professor of civil and environmental engineering, said in a statement.

The company that did the forecasting for the authority, Cambridge Systematics, is standing by its work, saying in a statement that "we emphatically disagree with the [Berkeley] conclusions that the ridership and revenue model is not reliable."