Authorities said Robert Root, 68, was seriously beaten at about 10:15 p.m. Wednesday.
Early Friday, Casanova Williamson, 19, was arrested on suspicion of assault with a deadly weapon and robbery in connection with the attack. Police said they are still searching for two other alleged attackers.
Officials said Root approached a group of young men who were being loud and profane on the train. He asked them to be quiet.
The men persisted in being loud. Police said Root approached the men again, only to be attacked and beaten.
Police said the attackers jumped off the train at the Alkali Flat station. The train continued to the 13th Street station, where it was met by police and an ambulance.