The individual was hit about a mile west of Oak Forest and later died, Metra spokesman Tom Miller said.
It was not immediately clear when the incident occurred, but the commuter rail tweeted about the collision just before 6:30 a.m.
"Extensive delays" on the line were anticipated, according to Metra.
Tinley Park and Oak Forest police did not have information about the incident Saturday morning.
Sun-Times Media contributed to this report.