A man beheaded a woman thought to be his mother, then jumped in front of a moving Long Island Rail Road train, in what’s being investigated as a brutal murder-suicide, law-enforcement authorities said. Officials found the body of a woman in her sixties outside a Farmingdale aparatment complex Tuesday night. Her head had been severed. The body of the man, who was in his thirties and believed to be the woman’s son, was found nearby, about a mile away on a section of LIRR tracks. He apparently had jumped in front of an eastbound LIRR train, officials said. The incident appeared to be domestic and not terrorism-related, according to authorities.