The great-grandmother mercilessly butchered allegedly by a homeless lunatic on an Atlanta train was carrying a knife — but didn’t have time to pull it out and try to defend herself, according to her grieving daughter.

Margaret Swan, 66, carried the knife for protection as she spent decades riding the Metropolitan Atlanta Rapid Transit Authority after never learning to drive, relying on the commuter rail every day before her throat was slashed and she was stabbed nearly two dozen times early Saturday.

“She took the train all the time,” Shanae Sams, 46, told 11Alive of her mother, a retired Atlanta Public Schools parent coordinator.

“You know everyone says to protect themselves, but my mom had protection on her. It was an act of nowhere,” she said of the horrific ambush. “She didn’t have time to even try to protect herself.”

Swan was sitting alone on the train around 11:20 a.m. when John Elijah Matthews, 25, was caught on chilling surveillance footage boarding and hovering over her before allegedly pulling out a knife and slitting her throat, according to a warrant obtained by the outlet.

The unwitting passenger — a mother of three, grandmother of five and great-grandmother of four — screamed and tried to escape, but the suspect allegedly held her down and stabbed her 18 to 20 times before leaving her dead on the floor in a large pool of her own blood.

“She was screaming for help, and nobody was helping her,” Sams told The Post on Monday.

Matthews was quickly arrested on the platform at Oakland City Station, still brandishing a knife.

The devastated daughter ripped local officials for temporarily opening the transit system to free riders, including the homeless, during systemwide renovations and failing to properly police its stations.

“To me, it’s negligence with the security and the patrolling,” Sams said.

“Anybody has access to get on and off the trains. If you’re allowing free stuff like that, for me, police patrolling was negligent, there was no security, there was nothing​. This whole situation is a situation that could have been prevented.”

Saturday’s ​fatal knifing happened less than a week after another MARTA rider was stabbed multiple times at the Georgia State Station near the Georgia Capitol last Sunday.

MARTA officials called the heinous daylight killing a “senseless and heartbreaking loss,” while noting that thousands of employees, 280 sworn officers and 30 field specialists work daily to protect riders.

“We are committed to [doing] everything we can to protect our customers and employees and respond quickly when incidents occur,” a transit spokesperson said Monday.

Matthews​, who was reportedly homeless​, faces one count of felony murder.

He is being held at Fulton County Jail.