It happened around 8:05 p.m. near Amity and Railroad Street.
According to a press release from Sgt. Tim Randall, both the engineer and the conductor saw the victim standing next to the tracks watching the train. They say the train was traveling about 25 miles per hour when the victim stepped in front of the train.
On Monday, the Canyon County Coroner identified the girl as Megan O'Day of Nampa.
The railroad tracks were shut down for about three hours. Nampa Police and Union Pacific personnel investigated the scene.