Building a high-speed train route over the Grapevine instead of through the Antelope Valley could save up to $4 billion, according to a July progress report released Wednesday.

A conceptual study identified more than one feasible alignment over the mountain pass, prompting engineers on the project to propose a more in-depth study of the Grapevine proposal, originally rejected in 2005.

But missing from the conceptual study, as of July anyway, was a close look at what effect a Grapevine route would have on the project's overall economics.

"More detailed analysis of ridership and revenue figures is required to complete the analysis between the Grapevine and Antelope Valley Alternatives," engineers with Parsons Brinckerhoff wrote in the July update released Wednesday by Bay Area critics of the project.

Board members of the California High-Speed Rail Authority are not expected to review the study's findings until October or November, at which point they could decide whether or not to launch a detailed study that would place the Grapevine in direct competition with the Palmdale route.

In May the rail authority approved the Grapevine conceptual study because that route could shave up to 9 minutes of travel time and 25 miles off a trip between Los Angeles and San Francisco. Also, recent engineering and environmental review work near Palmdale had identified unacceptable disruptions to the area and difficult design obstacles.

Any decision to focus more time and energy on the Grapevine could be politically risky. Government officials north and south of the Antelope Valley have backed the Palmdale option, which they say would support a growing population. Also, businesses in the area say they have made investments in expectation of a train station in Palmdale.

The city of Palmdale recently filed suit against the rail authority in an attempt to stop the conceptual study, which the city argued was forbidden under the project's funding stipulations. The suit was thrown out on jurisdictional grounds and the city has not said whether it will press forward with litigation.

Tejon Ranch Co., the Lebec-based agribusiness and real estate development company, has also opposed the Grapevine option for fear that such a route would interfere with its plans for a $2 billion mountain village.

At the same time, cost control has become a high priority for an agency under pressure to save taxpayer money even as virtually all estimates point to an increasing price tag for the project.

In 2005, the rail authority board rejected the Grapevine option after taking into consideration the route's challenging terrain and seismic conditions, potential impacts on farmland and fears that it would turn Bakersfield into a bedroom community of Los Angeles.

Construction on the project is set to begin next year; by 2020, trains running up to 220 mph are proposed to link Anaheim and San Francisco.