CHICAGO (WLS) -- A man was left bloody after he was punched while riding the CTA Pink Line.Police said the assault happened shortly after the victim got on at the Damen stop Tuesday morning around 10:30 a.m.Jim Balzer said another passenger was playing loud music on the train. When he asked him to turn it down, the man threatened him. He began beating him after Balzer hit the alert button to ring the conductor for help."I took a defensive position not knowing if this guy might have had a knife or a gun or something else, and next I looked up he was gone," Balzer said.CTA released a statement saying they are working closely with the Chicago Police Department in the investigation of the incident, including providing any available surveillance video from their camera system.