It happened just after 7 p.m. Wednesday, near the intersection of W. 120th Street and Crenshaw Boulevard.
According to the Los Angeles County Sheriff's Department, the victim, described as a Hispanic man between 20 and 25-years-old, was stabbed in the torso before dying at the hospital.
The suspect, identified as a Hispanic man between 35 and 40-years-old, was detained and taken to a local hospital for treatment of apparent gunshot wounds to his upper torso.
Authorities said the victim and suspect got into a fight while both were on the train and during that fight, the victim pulled out a gun and shot his attacker as the suspect began to stab him.
The suspect was able to get off the train but was arrested by deputies shortly afterward.
Bus shuttles replaced Metro C Line service between the Hawthorne/Lennox and Vermont/Athens Stations while police investigated, Metro reported.
The investigation remains ongoing.