The Sonoma County Transportation Authority's board of directors is being asked to chip in an additional $3 million to help get the first segment of a Sonoma-Marin commute rail line operating in three years.

The Sonoma-Marin Area Rail Transit district says it is about $21 million short of what is needed to open initial operating line from Railroad Square in Santa Rosa to downtown San Rafael in the fall of 2014.

Transportation planners have proposed that the Sonoma County Transportation Authority allocate $3 million, the Transportation Authority of Marin $8 million, and the Metropolitan Transportation Commission $10 million.

The proposal was drafted by the staffs of the three agencies.

"We are firm believers in providing passenger rail service in the North Bay," said Suzanne Smith, executive director of the Sonoma County authority. "It is in keeping with our general principle that we support passenger rail."

The Sonoma County authority's 12-member board of directors will discuss the funding on Monday.

Marin's transportation authority took up the issue on Thursday without any indication from directors of how they stand.

Marin County's proposed share of the funding shortfall is larger because Sonoma County already has committed $11 million to SMART and is hit harder by the agency's cost-cutting measures.

In Marin County, public support of the two-county rail district has been softer than in Sonoma County. The sales tax that supports SMART received approval from 60 percent of Marin voters, short of the two-thirds needed to pass, but overwhelming support in Sonoma County assured massage of the tax measure.

Nonetheless, passenger rail has been in Marin's transportation plans since 2001, said Dianne Steinhauser, executive director of the Transportation Authority of Marin.

"I don't know if our commitment or our legacy has changed. I think what has muddied the waters is that we are more actively supporting it financially," Steinhauser said. "The question is can we get there in a way that no other projects that are programmed are slighted; that we don't damage our other commitments."

Steinhauser said Marin can raise the $8 million by redirecting $6.5 million in federal transit funds and $1.5 million in state congestion management bond funds.

SMART is struggling to get a commute line started in the midst of the economic downturn, which has depressed sales tax revenues and hampered the amount of construction bonds that it can sell.

The construction of the Santa Rosa-San Rafael line is estimated to cost $470 million. SMART directors made $88 million in cost-saving measures, but are still $21 million short. The plan eventually calls for extending service south to Larkspur and north to Cloverdale.

SMART is expected to get bids for construction in July, along with a clearer indication of how much it can raise with the sale of construction bonds. MTC has asked SMART to delay selling bonds until SMART has a fully-funded plan in place.

The Sonoma County Transportation Authority meeting is at 2:30 p.m. Monday at the Sonoma County Permit and Resource Management Department.