Robert Little of Nanuet Community Ambulance Corps was quickly ushered from the scene when he realized who the victim was, emergency medical services officials said.
"There is no level of training that can prepare you for a scenario like that," Rockland County Fire and Emergency Services Director Gordon Wren Jr. told The (White Plains) Journal News.
"No one thinks that can happen, but it does. It's just incomprehensible," he said.
Little's son, Richard Little, had been walking in the middle of the tracks 35 miles northwest of New York City when he was struck by a southbound New Jersey Transit commuter train shortly after 8 a.m. Thursday, officials said.
The 32-year-old was wearing earbuds and sending text messages as the train approached from behind, the New York state Metropolitan Transportation Authority said.
He did not respond when the train engineer sounded the horn, the agency said.
Robert Little did not realize his son was involved when he responded to the emergency call, sources told The Journal News.