For two years, the California High-Speed Rail Authority said it could build 520 miles of high-speed train tracks between San Francisco and Los Angeles for about $43 billion.

But that figure -- long derided as unrealistic by critics -- went off the rails this month when the authority released detailed environmental reports for its proposed Merced-Fresno and Fresno-Bakersfield sections, the first two segments the agency wants to start building next year.

The authority's most optimistic estimates for the San Joaquin Valley sections alone total about $10 billion; route choices could run the price to $13.9 billion.

That's a far cry from the 2009 estimate of $8.1 billion.

If projected costs can rise by as much as 71% in the Valley -- a relatively flat, straightforward stretch -- what will happen when tracks must be built through mountains and across cities in the Bay Area or Southern California?

If costs escalate statewide as much as in the Valley, the price to build the system from San Francisco to Anaheim could leap from the 2009 estimate of $43 billion to as much as $67.3 billion, even before buying any trains.

Some critics are saying, "I told you so," and others worry about even more cost increases in the Valley and statewide before a decade of construction begins in late 2012, as planned.

"It is about time that more realistic numbers are being used," said Elizabeth Alexis, co-founder of Californians Advocating Responsible Rail Design, a group that has long doubted the authority's estimates.

Roelof van Ark, the rail authority's CEO, acknowledged last week that the earlier estimates, set forth in a 2009 business plan to the Legislature, were "a little bit optimistic."

Construction plans have changed in the Valley between 2009 and now, van Ark said.

He said that an updated plan due to the Legislature in October will reflect the higher costs for the Valley -- and statewide.

"What you're seeing in the Central Valley, you are going to see in the other parts of the state as well," van Ark said. "Quite a few of the components [that add to the cost in the Valley] will definitely carry into other parts of the state. However, some of them could be even larger."

Why so expensive?

The higher estimates in the draft environmental impact reports for the Valley segments are the result of engineers refining the route options and gaining a better understanding of construction challenges, van Ark said.

"We know more now," said van Ark, who was hired by the authority months after the 2009 plan was prepared. "When you start designing systems like this, you look at the alignment, the cities, the rural areas, and you make assumptions. ... [But] you don't have the detail to consider what real costs are going to come about."

With that detail in hand, the authority has identified about $5.8 billion in new costs, including:

About $3 billion more to build about 36 miles of elevated tracks over the cities of Madera, Chowchilla and Corcoran to avoid closing streets.

About $844 million more for elevated structures, tunnels, bridges, overpasses and undercrossings to cross waterways, streets, highways and railroads along the route.

About $685 million more for earthworks and retaining walls to raise the tracks above floodplains.

About $430 million more to purchase right of way along the route and to relocate displaced homes and businesses.

About $142 million more to realign a two-mile portion of Highway 99 in west-central Fresno to make room for the high-speed tracks.