Deputy Rudy Juarez, 50, suffered head injuries and broken bones after being hit about 6:15 a.m. along the Gold Line tracks, the sheriff's department said.
"He is unconscious but breathing on his own," Sheriff Lee Baca said in a statement. "Doctors have said that what happens during the next 24 hours is vital."
California Highway Patrol Sgt. Scott Morehouse told KCBS-TV Juarez was crossing the street under a freeway overpass when he was hit. He said the driver of the car that struck Juarez stopped and cooperated with investigators.
No arrest was made.
"It looks like it was purely accidental," he said.
Juarez is a 17-year department veteran assigned to the Transit Services Bureau.