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As the aftermath continued to play out, the most pressing question was how the Metrolink engineer could have missed the red signal.

KCBS-AM (740) reported Saturday that several teenage train aficionados said they had received a text message from the engineer shortly before the crash. The NTSB said it was treating the report with caution, noting that similar accounts had circulated after a crash in Boston but were found to be inaccurate.

"We've heard reports to that effect, but we have nothing to confirm," said NTSB board member Kitty Higgins.

The engineer was certified specifically on the L.A.-to-Simi Valley route, his regular assignment, and was familiar with signal locations, officials said.

The Union Pacific freight train and the 3:35 p.m. Metrolink train out of Union Station routinely passed each other near Chatsworth.

"That is a daily freight train. It's a regular traveler on those tracks," said Francisco Oaxaca, a Metrolink spokesman. He said the spot where the two trains pass can vary, depending on whether the freight train is running early or late.

"It was often either waiting in that area or we'd have to pull off and wait for it," said Mike Custodio, 37, an assistant city attorney who rides the 3:35 p.m. train on Fridays.

Shortly before the crash, the Metrolink train was stopped on a siding at the Chatsworth station. The red signal, apparently near the point where the commuter train returned to the single, shared track, was believed to be working properly, Tyrrell said. Those signals are controlled from the Metrolink dispatch center in Pomona, where train positions are constantly monitored.

The engineer is responsible for checking signals and abiding by them, Oaxaca said. Typically, when an engineer encounters a signal, he radios the train's conductor, who is supposed to radio back confirming the signal's color.

It wasn't clear if that procedure was followed Friday. "That's going to be part of our investigation and that's what we're working with the NTSB on," Oaxaca said.

L.A. Mayor Antonio Villaraigosa has asked local clergy to observe a moment of silence during Sunday prayer services to honor the victims of the crash and to pray for their families and the injured.

esmeralda.bermudez@ latimes.com

kate.linthicum@latimes.com

rich.connell@latimes.com