It happened shortly before 11:30 a.m. as the westbound Metro North train was traveling between Westport and East Norwalk, said Aaron Donovan, a Metropolitan Transportation Authority spokesman. The man, who was 45, was pronounced dead.
There were 150 passengers on the train at the time, he said. They were transferred to another train.
Service was suspended between South Norwalk and Southport section of Fairfield. It has since resumed.