Bay Area Rapid Transit workers walked off the job at midnight after six months of on-again, off-again negotiations fell apart. The move threatens chaos beginning Friday morning for the estimated 400,000 commuters who use the San Francisco Bay Area's largest commuter rail line.

This is the second BART strike, the nation's fifth biggest commuter line, in four months.

Union leaders on Thursday said they would walk out at midnight unless management agreed to enter arbitration to resolve remaining issues.

The impasse came after a marathon negotiating session that ran well into a second day Thursday as both sides worked toward a deal to end the months-long contract dispute that has left many riders under the repeated threat of a commute-crippling strike.



A team of federal mediators who participated in the talks for four days said the parties succeeded in agreeing to a number of significant items, such as salary, healthcare and pension contributions, but were ultimately "unable to bridge the gap'' on the work rules. BART unions had offered to have these rules be put in front of an arbitrator, but management did not want to.



"This is a classic example of a labor dispute that has implications far beyond that of the individual parties,'' said George Cohen, federal mediator. "Unfortunately, regrettably, we were not able to bring home the result we all want to achieve, which is a voluntary collective bargaining agreement.''



Roxanne Sanchez, president of Service Employees International Union Local 1021, said BART and its two largest unions were "extremely close'' to agreement on economic, health care and pension issues. However, she said the parties remained apart on work rule issues.

PHOTOS: BART Unions Threaten to Strike

She said the unions suggested taking the remaining issues to arbitration but management refused. Workers would walk off the job at midnight unless BART officials change their mind, Sanchez said.



BART General Manager Grace Crunican said the rules that the agency wanted to include in the contract were necessary to help managers run the system efficiently. She urged the union leaders to let their members vote on management's offer by Oct. 27.



"We hope to continue to work with the unions, but what's critical at this juncture is that unions take this offer to a vote,'' Crunican said.

BART Board President Tom Radulovich late Thursday asked the unions to return to the bargaining table. He said they are willing to negotiate wages and benefits if the unions are willing to negotiate workplace rules.

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Commuters have endured seven strike deadlines, sometimes staying up past midnight waiting to hear if the trains will run in the morning.

BART rider Leatrice Williams said she is tired of the uncertainty during labor negotiations. She since gave up on the train service and started riding the ferry.

In anticipation of a Friday strike, many commuters began planning for alternative ways to get to where they need to go.

"It's going to be chaotic coming from the East Bay," said Eddie Manganon, a Fremont resident. "It's going to be torture."



The contentious labor talks between BART and unions have dragged on for six months -- a period that has seen a chaotic dayslong strike, a cooling-off period and frazzled commuters wondering if they'll wake up to find the trains aren't running.



BART spokeswoman Alicia Trost said the agency has been flooded with calls and emails this week from commuters frustrated that they haven't been given earlier notices.



The key issues during most of the talks had been salaries and worker contributions to their health and pension plans.



Talks began in April, three months before the June 30 contract expirations, but both sides were far apart. The unions initially asked for 23.2 percent in raises over three years. BART countered with a four-year contract with 1 percent raises contingent on the agency meeting economic goals.



The unions contended that members made $100 million in concessions when they agreed to a deal in 2009 as BART faced a $310 million deficit. And they said they wanted their members to get their share of a $125 million operating surplus produced through increased ridership.

RAW VIDEO: Midnight Strike Threat Announcement

But the transit agency countered that it needed to control costs to help pay for new rail cars and other improvements.



On Sunday, Crunican presented a "last, best and final offer'' that includes an annual 3 percent raise over four years and requires workers to contribute 4 percent toward their pension and 9.5 percent toward medical benefits.



The value of BART's proposal is $57 million, Trost said, adding that the agency is looking at ways to incorporate the unions' counterproposals into that cost.



SEIU Local 1021 executive director Pete Castelli said Monday the parties were between $6 million to $10 million apart.



Workers represented by the two unions, including more than 2,300 mechanics, custodians, station agents, train operators and clerical staff, now average about $71,000 in base salary and $11,000 in overtime annually, the transit agency said. BART workers currently pay $92 a month for health care and contribute nothing toward their pensions.

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Meanwhile, Gov. Jerry Brown has stepped in to at least delay a strike by workers for regional bus system Alameda-Contra Costa Transit. Such a strike would leave commuters stranded without a mass transit alternative if a BART strike is underway simultaneously.



Brown appointed a three-member panel to investigate a strike notice by union workers. The move effectively prevents a strike, which had been threatened for Thursday, for a week. A 60-day cooling-off period in the contract dispute could then be imposed.

The Associated Press contributed to this report.