Saturday's accident on Bay Area Rapid Transit tracks in the East Bay city of Walnut Creek took place against the backdrop of a contentious and disruptive labor strike.
Two National Transportation Safety Board investigators were at the site of the accident on Sunday, NTSB spokesman Eric Weiss said.
The two-man team led by Jim Southworth, the board's railroad accident investigator-in-charge, will be looking at everything leading up to the collision, from safety procedures and qualifications of personnel to the track's condition.
"We will be the lead agency in the safety investigation into how and why this happened," Weiss said.
The four-car BART train with several people aboard was being run in automatic mode under computer control at the time of the accident, Assistant General Manager Paul Oversier said. The system has been shut down since Friday because of a work stoppage by the system's two largest unions.
The train was returning from a yard where workers cleaned graffiti from unused cars when it slammed into the two workers - one a BART employee and the other a contractor - who were inspecting an above-ground stretch of track between stations, Oversier said.
Neither BART nor the county coroner has released the names and ages of the victims. They were the sixth and seventh workers to die on the job in the system's 41-year history.
Following the May death of foreman who was killed by a passenger train in West Haven, Conn., the NTSB has been promoting improved safety measures for track maintenance crews, Weiss said.
In June, the board urged the Metro-North Railroad to provide backup protection for crews that were relying on dispatchers to close tracks while they are being worked on and to light the appropriate signals.
The investigators now in California will be checking to see if BART uses "shunts" - a device that crews can attach to the rails in a work zone that gives approaching trains a stop signal - or any other of the backup measures the NTSB recommended for the Metro-North system, Weiss said.
"Obviously, we are very concerned anytime anyone dies in transportation accidents, but we're very interested in the issue of track worker deaths right now," he said.
The fact that BART workers have been on strike since Friday would be part of the probe if it turns out to be relevant in terms of staffing and the experience and training of the track workers and train operators.
"We are not there because of the strike, but they would look at the circumstances and the personnel surrounding the issue," Weiss said.
Officials from the unions representing BART's train operators and some of the system's other workers have warned of the danger that could come with allowing managers to operate trains as BART had planned to do in case of a strike.
At a news conference Saturday, Oversier would not say whether a manager had been at the controls. In an earlier statement, BART said only that the person was an experienced operator. BART officials said on Sunday that they could no longer discuss the accident because of the ongoing NTSB investigation.
Meanwhile, with no indication that the striking BART workers would be back on the job Monday, the region was preparing for another day of gridlock on freeways and bridges clogged with commuters who would ordinarily be traveling by train. BART, the nation's fifth-largest commuter rail system, has an average weekday ridership of 400,000.
BART spokeswoman Alicia Trost said Sunday that transit officials and labor leaders have been in contact over the weekend, but the two sides did not have any plans to return to the bargaining table.
BART presented what it called it's last and final offer to its unions a week ago but is open to restarting the negotiations if that is what the federal mediator overseeing the process wants, Trost said. The system's directors plan to hold a special closed meeting on Monday, she said.
"The tragedy has redoubled everyone's commitment to a quick resolution so we can move forward in a spirit of cooperation to provide service to the Bay Area," she said.
A 2007 report by the U.S. Bureau of Labor Statistics found that between 1993 and 2002 a total of 460 railroad workers died on the job, 132 of them were pedestrian workers struck by trains and other rail vehicles. Of all the accidents, 62 involved local passenger trains.
Associated Press writer Andrew Dalton contributed to this report from Los Angeles. Cone reported from Fresno.