The unidentified girl reportedly left behind a suicide note before she walked into the path of an oncoming train at 10:45 p.m. Friday.
Police stepped up patrols in the area following the death.
The San Jose Mercury News said the victim would have started at Gunn High School as a freshman next week.
People stopped to let the train pass say they saw the child step onto the track, but could not get to her in time. The train's operator also saw her and blew his whistle, but could not stop in time.
You may remember last spring, two Gunn High School students died at the same crossing.
In May JP Blanchard, a junior at Gunn, walked in front of a Caltrain to his death.
A month later senior Sonya Raymakers also killed herself by standing in front of a train the week before she was supposed to graduate.
Just a few days later, a third student attempted to kill himself, but was stopped by his mother and a driver who noticed a commotion on the tracks and stopped to help.
The San Mateo County Sheriff's Department transit police are taking the lead on the latest investigation.
Caltrain spokewoman Christine Dunn said Caltrain will now work with a mental health task force. "This is a community issue that needs to be addressed," Dunn said.
Various resources are available for Santa Clara County residents experiencing a crisis, including a 24-hour suicide hotline that offers counseling over the phone. Many of the volunteers who staff the line have survived a loved one's suicide, said Gabby Olivarez, a senior mental health program specialist with the county and manager of the suicide prevention program.
The county also runs a suicide survivors' group where residents who have lost loved ones meet and support one another.
Olivarez said a basic mental health education can help parents and others recognize warning signs of suicidal behavior.
Those include disengaging from friends, family and social networks, losing interest in activities, and talk of suicidal thoughts. However, other signs can be less clear, she said.
"Feeling depressed, lack of sleep, no energy, fatigue: those are really signs of depression that can be linked to suicide," she said.
Teens who are still developing a sense of self-awareness often don't recognize they have a problem, she said.
"Younger people are very impulsive and more likely to do something and not think about it," she said.
However Olivarez said that in her experience, "there is always some kind of sign."
The hotline, which can see up to 100 calls a day, especially around the holidays, is taking many calls from people who have lost their jobs and struggling with financial issues, she said.
The service is staffed 24 hours a day. Residents in the north county can call 650-494-8420, and residents in the south county can call 408-683-2482.