Police said at least three people witnessed the collision, including the girl's father. Family members and friends were at the scene, comforting the girl's father Tuesday afternoon.
The collision happened just after 1 p.m. Tuesday. A freight train traveling eastbound hit the girl after she stepped across the tracks.
Police said her body flew 75 to 100 feet.
The collision happened near Placentia and Crowther avenues in Fullerton.
Police were interviewing witnesses, including the father of the girl and the girl's brother. The brother was sitting in the family car near the tracks facing the opposite direction.
Just seconds before the collision, the girl's father and younger brother were trying to persuade the 15-year-old victim to get into the family car. She had walked away from them while they were shopping nearby. Her father found her sitting near the tracks.
"They asked her to get in the car, and she refused. Rather than getting into an argument with her, they positioned themselves about 100 feet west of her, where they could kind of keep an eye on her," said Fullerton Police Lt. Alex Bastreri.
The lights flashed and the crossing gates came down as the freight train approached, heading east.
"She either tried to cross the tracks at that time, or she possibly intentionally stepped in front of the train," said Bastreri.
"Some witness statements that we've received said that she had been sitting there for quite some time, anywhere from five minutes to 20 minutes," said Bastreri.
The girl's identity was not released.