Caltrain's plan to ask voters for a sales-tax increase received a boost Tuesday, when Gov. Jerry Brown signed a bill that will allow the rail service to move ahead with its proposal.

Senate Bill 797, which was authored by Sen. Jerry Hill, D-San Mateo, authorizes a measure for a sales tax of no more than one-eighth of a cent. The measure would have to be approved by Caltrain's partner agencies in San Francisco, San Mateo and Santa Clara counties and then by at least two-thirds of the voters in the three counties.

Caltrain, which is in the midst of a modernization project that includes electrification of its fleet, has long been the only transit agency with no dedicated funding source in California. To date, it has relied on annual contributions from each of the three counties.

In a statement, Caltrain Executive Dirctor Jim Hartnett said Sen. Hill's bill is "critical to supporting our efforts to upgrade Caltrain service so we can meet the evolving mobility demands of our growing region."

"The Caltrain Electrification Project that is currently under construction will provide us with the potential to dramatically increase rail service to Caltrain communities, and SB 797 provides the opportunity to invest in that potential to truly meet the long-term needs of the region," Hartnett said.

Gov. Brown's signature brings the bill "one step closer to allowing the public to decide how they want to address traffic problems and improvements along the Caltrain corridor," Sen. Hill said in a statement.

"Our region is an economic powerhouse for our state and the Caltrain corridor is its major transportation artery," Sen. Hill said. "If our residents cannot get back and forth to work, school and their families because our main transportation corridor cannot accommodate them, we jeopardize the health of our robust economy and our quality of life."