A critical red light that a Metrolink train ran just before slamming into a freight train in Chatsworth was not as visible as green and yellow signals displayed by the same trackside warning device, investigators probing the disaster have found.

The clarity of the stop light, as well as possible violations of communication rules by the commuter train's crew, have become key focus points in the federal inquiry into the deadliest rail accident in modern California history.

"We did some signal inspections. Can't hardly see the red," one investigator told another just before the pair ducked into a closed-door briefing three days after the Sept. 12 crash. Now, similar observations have gained acceptance among investigators in the multi-agency inquiry, which is being led by the National Transportation Safety Board.

"It was the unanimous consensus of the investigative team that the red was not as illuminated or clear or clearly lit," said one knowledgeable source who requested anonymity because of restrictions on comments about the inquiry.

The reasons for the difference in visibility and what, if any, role it played in the crash could shed light on the central mystery of the catastrophe: Why did a veteran engineer barrel through a stop light that officials say should have been visible from a mile away?

Most public attention after the crash, which killed 25 and injured 135, focused on dozens of cellphone text messages received and sent by Metrolink engineer Robert M. Sanchez, who was also killed. One message was sent only 22 seconds before his train rammed into a Union Pacific freight train. It is unclear if a signal problem would reduce his potential responsibility for the crash.

NTSB officials note that train collisions normally have more than one cause. Agency investigators returned to Chatsworth in November to review signals and interview witnesses, The Times has learned.

A number of factors could have caused the red signal to be less bright, including circuit problems that reduce voltage to the bulb, according to experts who have investigated signal malfunctions.

"It happens from time to time, but not very often," said Bill Perry, a former Federal Railroad Administration signal inspector.

Metrolink has declined to comment on the investigation. But as part of a sweeping safety review after the crash, the agency's governing board accelerated a program to install hundreds more visible signal bulbs. No changes have been made thus far to the signal near the collision site, an agency spokesman said last week.

Experts say that text messaging, coupled with a light that was less clearly visible, could have caused a high level of "inattention blindness," resulting in Sanchez's failure to notice the warning signal.

"Your eyes are looking at it, but you don't see it," said David Strayer, a University of Utah psychology professor who has researched how using wireless communication devices affects motorists.

Failure to comply with trackside signals is a leading cause of rail accidents. On Nov. 20, a Metrolink train engineer went through a red light in Rialto and hit a freight train, injuring five passengers.

Part of the Chatsworth probe is zeroing in on a possible violation of a decade-old communication procedure intended to prevent signal-running collisions.

Engineers are required to call out via radio all signal colors and their locations as they become visible. Conductors, who typically ride several cars away from the engineer, are required to confirm all yellow or red signals -- those requiring the engineer to slow down or stop.

The importance of the practice was underscored last month when Metrolink tightened procedures after the Chatsworth crash. Engineers and conductors now must stop trains -- if necessary, using the emergency brake -- when one of them fails to call out or confirm a signal color. Previously, crew members were required to stop the train only after a stop or speed restriction was violated.

Announcing and acknowledging signals over the radio serves as an extra safety check for crew members, and it also alerts nearby trains that another vehicle is in the area, experts say.

"They are required to call those signals so they [all] are aware of what's coming," said former engineer Tim Smith, California chairman of the union that represented Sanchez. "It puts it on the entire crew, to understand the movement of the train."

Back-to-back train crashes in 1996 prompted new federal rules for passenger train crews to call out signals. The Maryland and New Jersey accidents, which together killed 12 people and injured 175, occurred after engineers ran red stop lights.