"Split schedules are something of great concern to us," said Kitty Higgins, a member of the National Transportation Safety Board, which is investigating the crash. "But whether that played a role in the accident is much too early to say."
Sanchez, who was killed in the crash, began his shift at 6 a.m. Friday, took a nap during a 3 1/2-hour break and resumed duty at 2 p.m., officials said. His train crashed about 2 1/2 hours later. Twenty-five people were killed and more than 135 others injured.
The findings followed tests by the National Transportation Safety Board that showed the two trains were only in each other's view four to five seconds before the collision. Investigators said the engineer from the Union Pacific freight train used his brakes about two seconds before the collision but was still going nearly as fast at the commuter train.
The NTSB has found the signals and tracks were working properly, and has narrowed its investigation to human error. The agency has subpoenaed Sanchez's cell phone records to determine whether he was text messaging before the crash.
On Tuesday, investigators interviewed the Metrolink conductor about Sanchez. The conductor said Sanchez told him he took a two-hour nap during his break but was not aware of any other factors that could have caused him to overlook the red light, Higgins said.
"He was not aware of any physical ailment that the engineer had," Higgins said. "He had no issues in his time of working with the engineer on the way he operated the train."
Higgins said the conductor confirmed that he and Sanchez did not call out and confirm the last two signals before the crash. She also said Sanchez routinely worked split shifts from 6 a.m. to 9 p.m. with 3 1/2 hours of break.
"It's a long day," Higgins said. "What the rules are and how this engineer or any crew member uses the time play a role into the accident."
Hours before regular rail service resumed on the stretch of track Tuesday, investigators conducted a visibility test to determine when the engineers in the crash would have been able to see each other in the moments before the collision.
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 the collision, the freight train was coming out of a tunnel, while the commuter train was rounding a bend. The test found the engineers had only seconds to react.
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."
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 the differing versions have not been reconciled.
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.
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."
In July 2007, Metrolink Chief Executive David Solow warned Congress that requiring the safety devices would involve "substantial cost," according to the Los Angeles Times. A Metrolink spokesman declined comment when called by The Associated Press.
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.
Associated Press writer Erica Werner in Washington contributed to this report.