Federal mediator George Cohen said late Tuesday progress has been made but has imposed a gag order on the parties.
The key issues have 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, offering 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.
But the transit agency countered that it needed to control costs to help pay for new rail cars and other improvements.