(Recasts with California governor intervening)
By Ronnie Cohen
SAN FRANCISCO, Aug 4 (Reuters) - California's Democratic governor intervened on Sunday to avert a San Francisco area rail strike that threatened to cripple the morning rush-hour commute, effectively prohibiting a walkout for seven days after the sides failed to reach a contract agreement.
Federal and state mediators had worked without success with negotiators for the area's train system, BART, and the two unions representing its workers late into Sunday night to try to reach a deal to avert a strike ahead of a midnight deadline.
To prevent travel chaos, Governor Jerry Brown appointed a three-member board to investigate the possible strike, and in a letter to both sides said state law would prohibit a walkout during the course of the week-long inquiry.
Brown's letter said he took the action "because the strike will significantly disrupt public transportation services and will endanger the public's health, safety, and welfare."
San Francisco area residents had been bracing for a strike even as 11th-hour talks continued, with some sleeping at friends' and relatives' homes to be closer to jobs and making alternative travel arrangements.
The unions gave a 72-hour notice on Thursday night of a possible strike by 2,400 workers on a rail system that carries 400,000 passengers a day and serves as a critical link in the San Francisco area's transportation system.
The unions say they are at loggerheads with management over wages, benefits and safety issues. They contend that despite a proposed 8 percent pay rise over four years, workers would still come home with less money after paying proposed increased contributions to pensions and health-insurance premiums.
Brown's action came after BART management asked him to declare a cooling off period to allow negotiations to continue without a disruption in the public transit system for at least 60 days, a BART statement said.
"The union is extremely disappointed," said Josie Mooney, a negotiator for one of the unions. She said that the unions and the public "waited for 22 hours on pins and needles" for a new contract offer but got nothing.
"We are deeply concerned," she said.
BART workers walked off the job in early July for the first time since 1997, before agreeing to a 30-day cooling-off period and returning to work after four days. They said they would strike again on Monday if a contract was not reached by midnight.
Last month's strike forced BART passengers to work from home, drive, carpool or crowd onto a limited number of buses and ferries for prolonged, frustrating commutes.
Traffic-choked highways, particularly across the Bay Bridge linking San Francisco to East Bay communities, left some in the generally labor friendly area upset with the strikers.
A poll by the Bay Area Council, a business-sponsored policy group, found 70 percent of 475 area residents polled opposed a strike, while 41 percent said they wanted the transit agency to use available resources to raise employee compensation.
The poll, conducted in early August, had a margin of error of plus or minus 4.5 percentage points.
Thomas Wong, 30, who rides BART between his Pleasant Hill home and his job in San Francisco, said he feels little sympathy for the workers.
"They're so much better off than a lot of other people in the Bay Area and around the country, and they're asking for more," he said, adding that his one-hour commute turned into a 2 1/2-hour nightmare during last month's strike.
Patrick Kallerman, 26, of Oakland, depends on BART to get to work as a policy director for a healthcare consulting firm.
"I would generally categorize myself as being on labor's side," he said. "I think they're getting a fair deal. They make more than I do."
The Bay Area Council Economic Institute estimated that July's strike cost the San Francisco Bay Area $73 million a day in lost worker productivity. (Editing by Dina Kyriakidou, Cynthia Johnston, Doina Chiacu and Pravin Char)