FREMONT — Construction of BART's long-awaited Warm Springs extension could start next summer now that transportation officials have secured all of the funds needed for the $890 million project.

The Metropolitan Transportation Commission approved a plan Wednesday to close the project's $144 million shortfall in part by loaning it $91 million that had been set aside for the already cash-strapped Dumbarton Rail project.

Although all of the funds for the Warm Springs extension have been pinned down, more than $200 million still is contingent on the Valley Transportation Authority committing to funding the BART extension into Santa Clara County.

The California Transportation Commission today is set to approve $239 million for BART to San Jose, fulfilling the state's promise made nine years ago to contribute $760 million for the 16.3-mile extension.

But even with state and regional agencies lining up solidly behind the $6.1 billion project, there is a still a huge missing link in the financial puzzle. The VTA needs about $750 million in federal assistance to build the BART line.

The focus now turns to the Nov. 4 election, in which Santa Clara County voters will be asked to support a one-eighth cent sales tax measure that would provide about $42 million a year to operate the train service.

Passage of the sales tax measure, which must be approved by a two-thirds vote, would make it easier to fulfill the funding condition for the Warm Springs extension, but it still could be satisfied even if the measure fails, said Jim Pierson, Fremont's director of transportation and operations.

Wednesday's MTC vote marked a landmark for the Warm Springs extension, which has been a top Fremont transit priority for three decades. Conversely it was a major blow for Dumbarton Rail, which Union City has touted as a vital ingredient for transforming its BART station into a regional transportation hub.

The Dumbarton project, which would include a new rail line with trains running across the Bay between Union City and both San Jose and San Francisco, became imperiled two years ago when project costs essentially doubled from $300 million to $595 million.

With the Warm Springs extension much further along in terms of planning and financing, transportation officials decided to loan it $91 million of the $300 million that had been allotted for Dumbarton Rail.

The cash infusion gives the Warm Springs project money that will be available when construction is expected to get under way during the next two years. Some of the funds that had been anticipated, namely revenue from BART's San Francisco airport extension, have yet to materialize.

The $91 million is to be paid back to the Dumbarton project by 2019, but there is no condition that it be paid back with interest.

"Certainly, we would have liked to have seen a tighter program for payback of the loan," said Mark Evanoff, Union City's redevelopment manager.

The MTC still plans to fully fund Dumbarton Rail. It included a provision in Wednesday's vote that it would look for funding to help acquire the Oakland Subdivision, a stretch of rail track in Southern Alameda County owned by Union Pacific. If the subdivision could be acquired and dedicated to passenger rail lines, including Altamont Commuter Express, Capitol Corridor and Dumbarton, that would help pave the way for Union City's BART station becoming a transit hub.

BART still is unlikely to begin construction of the Warm Springs extension until the Santa Clara funding condition is satisfied, Pierson said. If the sales tax measure is approved, BART could advertise the construction in February and begin construction in June.

Under that schedule, Pierson said, the Warm Springs BART station would be operational in mid-2014. The Oakland A's are hoping to move into a new stadium in Fremont less than two miles from the station in 2012.

The MTC vote included a provision that in six months the commission will review the plan for getting BART to San Jose. However, Alameda County Supervisor Scott Haggerty, who sits on the commission, said there were no plans to return the $91 million to Dumbarton rail if the BART extension stalls.

"We want to move forward with projects and get them built," he said. "Getting BART down into San Jose is long overdue."

Gary Richards of the San Jose Mercury News contributed to this report.

Fremont reporter Matthew Artz can be reached at 510-353-7002 or martz@bayareanewsgroup.com. Read his blog posts at www.ibabuzz.com/tricitybeat/.