The incident happened at the Willowbrook/Rosa Parks Station, 12 miles south of downtown, around 7:41 p.m. PT, The Los Angeles Times reported.
All four victims were taken to a hospital, and at least three were in critical condition, according to the newspaper.
"He shot three times! Then more arguing and pushing and shoving," witness Andrew Maynard told KNBC-TV in Los Angeles He said it shooting started with an argument between several men. "He shot three more times and a man ran towards me to get away. They shot him in the leg! He collapsed next to me! I was crouched down trying to hide."
No arrests were immediately reported. There may be more than one suspect, a spokesperson for the L.A. County Sheriff’s Department’s Transit Services Bureau said, according to The Times.