The effort to bridge a $21 million financial gap for the initial Sonoma-Marin commute rail line is taking on a new urgency, with the rail district getting ready to sell bonds and seek construction bids.

Negotiations were held last week by the Sonoma County Transportation Authority, the Transportation Authority of Marin and the Metropolitan Transportation Commission on sharing that final piece of funding.

MTC, the regional transportation agency, is working with the Sonoma and Marin transit bodies "on trying to find the money to close this gap," said Randy Rentschler, MTC's manager of legislative and public affairs.

"How much, how are we going to share, a third, a third, a third? I cannot tell you what the number is. We are talking about it, but we have not put money on the table."

The issue will be discussed Monday by Sonoma's transportation authority, where the feeling is Marin has not done its share.

On Thursday, the issue will come before Marin's transportation authority in what could be a contentious meeting because Marin voters gave the rail plan only lukewarm support in the November 2008 election.

The Sonoma-Marin Area Rail Transportation agency, SMART, is struggling to get a commute line started in the midst of an economic downturn that has depressed sales tax revenues that are needed to back construction bonds.

MTC has asked SMART to postpone the sale of construction bonds until it has a financial plan that covers the complete cost.

The regional planning agency holds the purse strings for regional, state and federal funds for the nine Bay Area counties, including the $22 million in toll bridge funds in Marin County that have already have been promised to SMART.

MTC has also taken a stronger role in getting the SMART project into construction. It is similar to MTC's last-minute addition of $20 million for a BART connector to the Oakland Airport last year and $80 million to the Doyle Drive approach to the Golden Gate Bridge in 2009.

"One of the things we do here is try to get projects across the finish line," Rentschler said. "There is no doubt that SMART is in that neighborhood. It has a lot of money associated with it. We know it is close -- the last mile is the hardest."

SMART's latest plan is to initially construct a line from Railroad Square in downtown Santa Rosa to downtown San Rafael, scheduled to open in 2014.

The rail agency wants to bid for construction bonds in mid June. The exact cost of construction and how much SMART can sell in bonds would be known in July.

The construction cost initially was estimated to be $470 million, or $109 million more than SMART expects in bond sales and has available in other local, state and federal funds.

SMART directors have approved $88 million in cost-cutting measures, which included deferring stations at Corona Road in Petaluma and Atherton Road in Novato and purchasing fewer trains.

Dianne Steinhauser, executive director of the Marin transportation authority, said her board will not take action on Thursday, "but it will talk about what shares are being proposed for the three agencies, and then what principles and what conditions we want to move forward on."

In Sonoma County, however, the feeling is that Marin should put in the most money.

Sonoma County's transportation tax already allocates $11 million to SMART, and the county is also taking the brunt of SMART's cost-saving measures.

"I think we will put in more money, but the heavy lifting has to be by Marin," said Valerie Brown, SMART chairwoman and a Sonoma County supervisor.

You can reach Staff Writer Bob Norberg at 521-5206 or bob.norberg