The incident occurred at about 11:20 a.m. just east of the Westport train station, said Metro-North spokesman Aaron Donovan.
Why the person was on the tracks is a question the investigators are looking into, he said.
"Metro-North police are investigating what led to this person walking on these tracks," Donovan said.
He was struck on the eastbound tracks, he said.
The trains were briefly stopped but are now running on the New Haven Line with a 15- to 40-minute delay as the investigation continued, Donovan said.
No further information was available on the victim about his age or hometown, he said.
The train was the 10:04 a.m. from Grand Central bound for New Haven at 12:10 p.m., Donovan said.
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