By Mike Rosenberg

BART’s unions Thursday evening filed a 72-hour strike notice, sharply escalating the threat of a work stoppage on Monday that would once again paralyze Bay Area commutes if an agreement is not reached on the key issues of pay raises and contributions to health care and pensions.

The threat left many frustrated commuters with the question: Can’t someone save us?

Despite the formal notice, a strike is not guaranteed. Both union and management leaders say they will spend the next three days bargaining in hopes of reaching a deal. And there are other options for both sides.

Among the alternatives that have prevented BART service stoppages during tense labor talks over the past few decades: Unions could simply continue to work without a contract; BART could bring in back-up operators; the governor could step in to halt a shutdown; or BART could turn up the pressure by imposing a contract.

None of those options is being seriously considered publicly now, though leaders on both sides have indicated that might change as negotiations head into the stretch.

BART is suggesting that unions continue the agreement that temporarily ended the first, 4½-day shutdown on July 5 and set the Sunday night deadline for a long-term contract. Under that deal, employees stayed on the job under the terms of their old contract — a common practice among public and private union workers that allows workers to continue cashing paychecks.

Most recently, AC Transit workers, who had previously threatened to strike, continued to work after their contract expired at the end of June. And BART unions say they’re open to following suit.

“That’s a decision that we will assess on Sunday night,” said Josie Mooney, chief negotiator for the largest labor group, the local Service Employees International Union.

But Mooney said BART unions have worked without a contract only when a deal appeared imminent — including in 2005, to avert a strike — and that’s not yet the case this time.

Labor experts, though, say working without a contract may be the best strategy for unions as they continue to struggle to gain the public support needed to apply pressure on management.

“I think the BART employees need to be careful not to overplay their hand here,” said Joe Nation, a public policy professor at Stanford and former union representative. “If (the public) is not sympathetic to striking workers, I think it makes it much tougher for that strike to be successful. And management will see that. That’s a troubling position for the union to be in.”

On Thursday, BART and unions held dueling press events — including a worker march to BART’s Oakland headquarters — in which they yet again bashed one another, even while spending part of the day at the bargaining table.

BART’s union workers have not received a pay hike for four years. But they make an average of about $76,500 in annual gross pay, contribute nothing toward their pension and pay $92 monthly toward health care. The sides remain several percentage points apart on increases to pay and benefits contributions.

If the union’s 2,300 blue-collar workers hit the picket lines, BART could use its 10 or more certified managers to run trains on a limited basis, as it did during a three-month strike in 1979, or bring in retired train operators.

But BART spokeswoman Alicia Trost said the agency, for now, is focused on a plan to use 95 charter buses, slightly more than during the first strike, to take a couple of thousand displaced passengers from the East Bay to San Francisco each day of a strike.

If the work stoppage wears on, though, BART could change course and consider running some trains. “It depends on how long the strike would go, and resources and staffing,” Trost said.

BART cannot bring in an entirely new crew of train drivers since the union contract prevents the agency from beginning the state-required training process — which can take months — until workers go on strike.

Another option is for Gov. Jerry Brown to use his powers to issue a 60-day cooling-off period, a route several previous governors have taken to avoid BART strikes over the last few decades — most recently Gov. Gray Davis in 2001.

A spokesman for Brown, however, said the governor’s office has not yet received a request for such action from either BART or the unions. And neither Trost nor Mooney on Thursday indicated any willingness to ask Brown to step in.

In the run-up to the last strike in late June, the other union — the local Amalgamated Transit Union — asked Brown to declare a cooling-off period. But he declined after BART management argued that if there had to be a strike, it would rather have one during a light holiday week.

BART could also try pushing unions toward a deal by imposing terms of employment, as it did during the last round of contract talks, in 2009, when workers also threatened to strike. But BART board meetings to discuss that option in July were canceled after both sides agreed to the deal to temporarily end the strike, and the agreement prevented the board from imposing work conditions until at least Aug. 5.

Despite the possible alternatives, both sides say they remain committed to the most straightforward solution: reaching a settlement.

“That’s what we want; we want an agreement. Workers don’t want to go on strike,” Mooney said. “We appreciate the impact it has on the (riders) so they can get to work and provide for their families.”

Contact Mike Rosenberg at 408-920-5705. Follow him at twitter.com/RosenbergMerc.