A female victim reported to the station agent at the 12th Street Station that a male had been masturbating while she was on the Richmond bound train between Embarcadero and West Oakland Stations. An officer responded to take a statement from her and an area check for the suspect (who had also exited at the 12th Street Station) was conducted, but he was not located. A light duty employee checking the surveillance cameras noticed a male at the station who matched the description. The suspect was contacted by an officer as he was about to board a train. The victim positively identified him as the person she had seen masturbating earlier and placed him under citizen's arrest. The male was issued a prohibition order and booked into the Glenn Dyer Jail for felony indecent exposure (prior convictions for same offense) and for a parole violation. Train video was requested.