Lynnsey Evakarla Braun, 32, is accused of fatally shooting a man in his 50s inside an outbound Gold Line train Thursday night, the Sacramento Bee reported.
Authorities said Braun shot the victim, whose name was not reported, three times when the train arrived at a station. The victim, who exited the train before collapsing, was pronounced dead at the scene.
Two other people were in the train car at the time of the shooting, but were not injured.
Braun was arrested about 12 hours later on suspicion of murder.
Sacramento police Lt. Norm Leong said investigators have reviewed surveillance footage of the incident and said it appeared Braun and the victim exchanged a few words before the shooting, but likely didn't know each other.
The nature of the conversation remains unclear, Leong said.