SAN FRANCISCO (REUTERS) - Talks to avert a potentially crippling San Francisco area rail strike failed to produce a deal by a Monday midnight deadline, but the sides will continue to meet through the night and trains will run on Tuesday, a federal mediator said.

Unions representing Bay Area transit workers spent the day negotiating with management, with union leaders saying only an 11th-hour "hail Mary" would likely stop a walkout.

But a federal mediator involved in the talks said early on Tuesday some "constructive and productive progress" had been made.

"Because of all of our concern about the public's interest, I am authorised to announce that trains will run tomorrow (Tuesday)," Mr George Cohen, director of the US Federal Mediation and Conciliation Service, told reporters. About 400,000 passengers use the rail system every day.

More than 2,000 drivers and other employees at Bay Area Rapid Transit (BART) have been working without a contract since June 30, and two unions representing them had promised to walk off the job on Tuesday if no deal was reached.

Management has offered a 12 per cent pay raise over four years for workers, who they say make an average of US$79,000 plus benefits. The unions peg the average worker salary lower at US$64,000, excluding manager pay.

"We think we have a great offer on the table. We don't think a strike is necessary. We don't think a strike is smart," said Mr Tom Radulovich, president of the BART board of directors. "So far they're not excited about our last, best and final (offer), but they are talking to us."

Mr Peter Castelli, executive director of Service Employees International Union local 1021, has said negotiators were close to a deal but time was running out. He said unions had made a counter offer, but the details were under wraps.

Should a strike halt the BART trains, riders would be forced to turn to the bus system for public transit options. However, a union representing workers at the Alameda-Contra Costa Transit Agency, the third largest in the Bay Area, on Monday notified its management they could strike as soon as Thursday.

Partly because BART worker pay is considered by many to be generous - and considerably higher than the median US salary of about $50,000 - the potential strike comes against an unusual backdrop of public discomfort with a possible labour action in a typically pro-union region.

"The Bay Area traditionally is a very pro-labour part of California. But this go-round the issue seems to be framed differently," said Larry Gerston, a retired professor of political science at San Jose State University.

"The relatively high salaries of BART employees, the overtime they routinely get in conjunction with lots of sick time, and that's against a backdrop of a public that's just recovering now from a recession where every dollar meant a whole lot."

The unions point out that San Francisco and nearby Oakland are both among the 10 most expensive US cities. Because the management offer also includes a demand that employees pay a portion of their medical and retirement benefit costs, some of the 12 per cent offered in raises would be immediately swallowed up by those contributions.

The unions had initially asked for a three-year contract, with a 3.75 per cent raise in each of the first two years and a 4 per cent raise in the last year.

BART spokesman Rick Rice has said the transit system wanted workers to contribute to pensions, starting at 1 per cent in the first year and growing to 4 per cent in the fourth.

The agency also wanted a cap on its health care costs, he said.

Bay Area commuters had a taste of the havoc a transit strike could bring in July, when BART employees walked off the job after their contract first expired. A strike was again threatened in August before Democratic Governor Jerry Brown intervened to seek a 60-day cooling-off period, now expired.

Mr Melvin Mendoza, 31, had to take three days off from his job as a technical support specialist at a San Francisco law firm during the July action.

"Transportation last time was just a nightmare," said Mr Mendoza, a father of two whose wife uses the family's only car to get to her job. "The way this is going, it's putting a bad taste in my mouth both on the part of BART and with my concept of the unions."