OAKLAND  (AP) — Striking San Francisco Bay Area rail workers and transit agency officials returned to the bargaining table on Thursday for talks that one union leader described as “frustrating.”

Joe Bomberger with the Service Employees International Union told reporters as he entered the negotiating site in Oakland that Bay Area Rapid Transit officials were not “substantially addressing” any of the safety concerns that the unions have for the public and workers.

Those concerns include lighting in tunnels and bullet-proof glass for station agent booths.

Bomberger was then pulled away by another union official. The two sides have been told by a state mediator not to speak to the media.

The strike is in its fourth day, though commuters got a reprieve from crowded buses and gridlock on the roads because of the Fourth of July holiday.

BART is the nation’s fifth largest rail system and carries about 400,000 commuters each weekday.

The two sides negotiated into the night Wednesday. BART issued a statement, saying it was sorry that the actions of the unions had caused such a tremendous disruption.

The strike began early Monday after talks broke off. Negotiations resumed Tuesday as political pressure and public pleas mounted.

Key issues in the labor dispute include salaries, pensions, health care and safety.

BART has said workers from the two unions average about $71,000 in base salary and $11,000 in overtime annually. The workers also pay a flat $92 monthly fee for health insurance.

The unions — which represent nearly 2,400 train operators, station agents, mechanics, maintenance workers and professional staff — want a 5 percent raise each year over the next three years.

BART said it is offering an 8 percent salary increase over the next four years as well as reducing the amount of employee contributions it originally requested for pension and medical benefits.