The man was walking east on the tracks in the 100 block of East Santa Fe Avenue when he was hit by the eastbound train about 4:45 a.m., Fullerton police Lt. Lorraine Jones said. He died at the scene.
Passenger trains continue to run in the area, but there were some delays.
Fullerton police Sgt. Andrew Goodrich said the fatally injured man was a local transient in his late 20s. His name was withheld, pending notification of his relatives.
The death was being investigated as an apparent accident, Goodrich said.