OAKLAND -- In addition to temporary bus bridges to aid commuters, if a BART strike becomes "extremely lengthy" the agency has a dozen managers who could shuttle riders on the trains, the agency said Saturday as talks seemed to be making progress in the attempt to avoid a Monday morning strike.

Lead union negotiators left the building just after 10 p.m., leaving behind some junior team members to go over cost details with BART management, but they too soon began trickling out.

"Frankly, we have a long day tomorrow," said Pete Castelli, executive director of SEIU Local 1021.

BART's chief negotiator, Thomas Hock, said from his standpoint, the sides are "a lot more than halfway there."

Using BART management to operate trains is a last resort, BART spokeswoman Alicia Trost said Saturday, but the 12 former train operators are capable of operating them and would do¿ so "only if a strike becomes extremely lengthy." Such a move would have to be approved by BART's board of directors, and union officials have already attacked such a plan, saying it would endanger riders.

BART has scheduled 130 to 150 buses to arrive Monday and pick up commuters at nine East Bay BART stations and take them to San Francisco in the event workers walk off the job that morning, Trost said.

Both sides spent part of Saturday caucusing separately, Castelli said.

"It's important work," he said. "Committee work leads to proposals. We're trying to finish up supplementals, smaller issues. We expect to be here a long time tonight."

BART General Manager Grace Crunican, who entered talks Friday, was off-site for most of Saturday, Trost said, but Crunican and Hock returned to the building at 6:30 p.m.

Neither side is discussing specific bargaining numbers, honoring a gag order requested by the mediator. But Hock did emerge from the building shortly before 8 p.m. to report that while the most critical pay and benefit disputes still needed to be resolved, the two sides had tentatively agreed to some supplemental items requested by SEIU.

"We're still hoping we can get this done," Hock said. "Whether or not we do, whether or not we don't, that's up to them."

He characterized the tenor of the talks as cordial. "Everyone's very pleasant," he said.

Chris Finn, a BART train operator and recording secretary for Amalgamated Transit Union Local 1555, spoke to reporters about a half-hour later.

The unions, he said, had "presented some new ideas to management, and we'll see how they're responsive to those."

After lead negotiators met earlier in the evening, smaller committees would be convening late into Saturday night "to see if we can agree on the numbers," Finn said.

Monday's deadline is the fifth work stoppage deadline of the past few months.

First, unions called a strike in the wee hours of July 1, shutting down trains before most commuters woke up. Then management asked the governor to avert a shutdown late at night on Aug. 4. After that, a judge ordered the cooling-off period at the governor's request on the last day possible, Aug. 11.

Polls suggest commuters have had enough, with 76 percent of Bay Area residents opposed to a strike, according to a SurveyUSA poll commissioned this past week by KPIX.

Staff writer Mike Rosenberg contributed to this report. Contact Matthias Gafni at 925-952-5026. Follow him at Twitter.com/mgafni.