A female reported a male subject was masturbating on A San Francisco Airport bound train between Pittsburg/Bay Point and North Concord/Martinez stations. The officers contacted the victim aboard the train who positively identified the suspect and placed him under citizen’s arrest. The suspect was on misdemeanor probation out of Alameda County. The suspect was transported to the Martinez Detention Facility where he was booked for indecent exposure and a probation violation. The suspect was also issued a prohibition order prior to being booked into jail.