OAKLAND -- BART will focus its environmental planning for a rail extension to Livermore on a first segment along Interstate 580 to Isabel Avenue.

BART board members said Thursday that they will hold off on environmental studies of any other Livermore site that would require a decision on whether to stay along the freeway or lay tracks to downtown.

Board members and planners delivered that message in their meeting Thursday night to Livermore leaders, who wanted the rail system to pledge it would never study the downtown alignment as an option.

BART's answer: It's premature to bar a route for a segment BART won't even do environmental studies on for years.

A BART train speeds along Highway 24 on Oct. 9, 2013, in Orinda.
A BART train speeds along Highway 24 on Oct. 9, 2013, in Orinda. (D. Ross Cameron/Staff file)

"It's not the right time to exclude an option, even though I strongly agree BART should not go to the downtown," said John McPartland, a board member from Castro Valley.

For now, BART officials said they are studying alternatives for a 5-mile, $1.2 billion rail extension from the end of the line at the Dublin-Pleasanton station to a proposed new station near Isabel Avenue.

To meet environmental laws requiring an examination of alternatives, BART also is studying options for express bus service and a diesel train, both less expensive than regular BART service.

The environmental review is expected to be complete in 15 to 18 months, setting the stage for the BART board to approve a project and then seek funds to build it.

BART would get $400 million for the first phase of the Livermore extension if Alameda County voters approve a November ballot measure to double the county transportation sales tax from a half-cent on the dollar to a full cent.

Livermore Mayor John Marchand asked the board to swear off any consideration of the downtown route, which is opposed by the Livermore council and 8,000 local residents who signed a petition.

Marchand said he is concerned that leaving the door open for a downtown route could hurt the political campaign for the county sales tax increase in November.

BART officials, however, said it would be premature to oppose an option before it is studied, as required by state law.

"If you haven't studied all your options, you haven't done your job," said Director Tom Radulovich, of San Francisco. "You leave yourself open to legal challenges.

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff.