Betti was hit by a Burlington Northern Santa Fe train at around 5 p.m. on Sunday as she hung out by the railroad tracks near Old Orchard and Howe roads.
A railway spokeswoman said the girl was on the tracks with another person, but that the two moved when they saw a train approaching. Betti later returned to the track and was hit.
She added the train was heading from Richmond to Fresno, and had three crewmembers aboard at the time of the incident.
In a message posted to Facebook, Betti's mother Dena Betti explained her daughter had been at the tracks with her boyfriend and that she had returned after realizing she had dropped her phone.
"She didn't judge the approach, and the train creates a vacuum, we were told, and it sucks you in...," Betti's mother wrote.
Around 1,000 friends, classmates and teachers attended Tuesday's vigil in Betti's name. The school secretary said that grief counselors had been at the school and will remain there throughout this week.
Betti was a soccer player and had two younger sisters.