The incident was reported at about 9:10 a.m., BART spokesman Jim Allison said. The victim was under a train, he said.
The San Mateo County coroner's office has responded to the scene, and BART police are investigating the death, Allison said.
There was no immediate information about the victim's age or sex.
Trains are still operating through that area in both directions but are not stopping at the Colma station, Allison said at about 10 a.m. Passengers are advised to use the Daly City or South San Francisco stations instead.
The incident initially caused delays of 15 to 20 minutes, but Allison said trains are back on schedule.