Authorities said the train cars were being relocated when they derailed, partially burying the man, WESH-TV, Orlando, reported.
A locomotive was pushing 10 cars hauling gravel to be used in a railway construction project when the four cars came off the tracks just before 6:40 a.m., said Steve Olson, a spokesman for the Florida Department of Transportation.
The train "was moving at a slow rate of speed, and it was going to be relocated," Seminole County Fire Department spokeswoman Paula Thompson said. "To where I don't know or how far. I'm not sure at this point."
The body of the man was found pinned under an overturned car as heavy equipment operators removed the rubble.
The man, who worked for the contractor delivering the gravel, was watching the train move down the tracks, said a fire department spokesman.
The accident did not occur on a main line and did not affect train traffic.
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