Only a few people boarded the commuter train in Chatsworth, and just three sat in the first car, the compartment that was the most damaged in Friday's collision.
Among those in the first car was regular rider Sal Garcia, who said he had been sick Friday and stayed home. Sitting in his usual seat was therapeutic, he said.
"It was really emotional. I sat down and started to cry, and that made me feel a lot better," Garcia said.
Also in the first car was LeeAnn Clifton, a visitor from northern California.
"This is a little eerie," she said.
The train was monitored closely by rail workers as it slowly passed the site of the crash. Residents along the route left their homes and waved as the train passed. Flowers were laid along the platform at the Simi Valley station.
Hours before service resumed, investigators conducted a visibility test to determine when the engineers involved the crash would have been able to see each other in the moments before the nation's deadliest rail disaster in 15 years.
A Metrolink train and a Union Pacific locomotive were brought nose to nose on the tracks where the crash occurred. Investigators then backed the stand-in trains away from each other.
In the moments before Friday's collision, a Union Pacific freight train had exited a tunnel, while the commuter train was rounding a horseshoe bend.
"When did one come into view with the other?" Kitty Higgins, a board member of the National Transportation Safety Board, said before the test. "What will that tell us?"
NTSB officials said it would be the final test conducted at the site. The agency did not immediately return phone messages seeking comment about the results of the test.
One test observer was Lilly Varghese, a friend of 57-year-old victim Beverly Mosley.
"I came here to pay respect to where I lost her," Varghese said. "She lost her soul here."
Varghese said she and Mosley worked together as nurses in the prenatal unit of a hospital. Mosley had two adult daughters and had become a grandmother about seven months ago.
The Metrolink commuter rail service has blamed the crash on the failure of its engineer to stop for a red signal, but the NTSB has withheld judgment and said its investigation will take months to complete.
In Washington, Sen. Dianne Feinstein introduced legislation Tuesday requiring the installation of technology to prevent train crashes and warned that there would be more disasters without it.
The California Democrat hopes to nudge Congress to pass her requirement for so-called positive train control before recessing at the end of next week. The House and Senate have already passed separate legislation to implement the technology but time is running out to reconcile the differing versions.
The technology can engage the brakes if a train misses a signal or gets off track. It has been installed on a fraction of U.S. rail tracks but not on the one where Friday's crash occurred.
Feinstein blamed "a resistance in the railroad community in America" to the price tag of installing the systems.
Failure to act now, she said, amounts to "negligence, and I'll even go as far to say I believe it's criminal negligence not to do so."
Resolutions supporting the technology were also introduced by members of the Los Angeles City Council and the county Board of Supervisors.
The Association of American Railroads, the lobbying arm for the freight railroads, has said it does not oppose the legislation but is concerned that the technology has not been perfected.
Meanwhile, federal investigators were continuing to look into whether the engineer of the Metrolink commuter train was text messaging on a cell phone before Friday's deadly wreck. The engineer, Robert Sanchez, was killed in the collision.
Investigators with the NTSB did not find a cell phone belonging to Sanchez in the wreckage, but two teenage train buffs who befriended him told KCBS-TV that they received a text message from him a minute before the crash.
Higgins said the NTSB issued a subpoena to get the engineer's cell phone records. She said Verizon Wireless had five days to respond to the demand.
The state's top rail safety regulator is seeking an emergency order banning train operators from using cell phones.
"Some railroad operators may have policies prohibiting the personal use of such devices, but they're widely ignored," Michael Peevey, president of the California Public Utilities Commission, said Monday.
The commission is scheduled to vote on the order Thursday.
Metrolink prohibits rail workers from using cell phones on the job, but federal regulations do not address the issue, Federal Railroad Administration spokesman Steven Kulm said.
In 2003, the NTSB recommended that the FRA regulate the use of cell phones by railroad employees on duty after finding that a coal train engineer's phone use contributed to a May 2002 accident in which two freight trains collided head-on in Texas. The coal train engineer was killed and the conductor and engineer of the other train were critically injured.
