The devastated daughter of the Atlanta train passenger viciously slaughtered by an alleged homeless maniac in a random attack ​ripped local officials​ Monday for opening the transit system to free riders​ — and then failing to properly police ​its stations.

Margaret Swan, ​a 66-year-old great-grandmother, had her throat slashed and then was stabbed 18 to 20 times on the Metropolitan Atlanta Rapid Transit Authority by the knife-wielding madman in an unprovoked attack Saturday morning, cops said.

Shanae Sams​, Swan’s daughter, told The Post that glaring security gaps, lax policing and a temporary free-fare policy on MARTA paved the way for the heinous daylight killing.

“To me, it’s negligence with the security and the patrolling,” the 46-year-old ​woman said, noting how ​commuters were given free rides because of renovations across the transit system​.

“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​,” she said.

“The whole situation is a situation that could have been prevented.”

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 11Alive.

The ​victim — 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 another 18 to 20 times before leaving her dead on the ground in a large pool of her own blood.

“She was screaming for help​, and nobody was helping her,” Sams said.

“There’s no words that can explain how we feel right now. We are all lost, confused, angry and scared. It is just too much. Why was there no security? Why is nobody trying to prevent this from happening?”​ the daughter said.

After the bloodbath, Matthews hurled Swan​’s lifeless body to the floor and stood over her as the train pulled into Oakland City Station, where he was quickly busted on the platform still brandishing a knife, the warrant said.

H​e​ was arrested after horrified riders described his clothing to MARTA police. 

“I screamed, cried and hollered,” the grieving daughter said of the moment she learned her mother, a retired ​parent coordinator ​with Atlanta Public Schools, had been killed.

“It’s hard. My mom was the rock of our family​, and now we don’t have that foundation.​” Sams said. “She was a very loving person. She would help anybody that she could. There wasn’t a bad bone in her body.”

Matthews​, who was reportedly homeless​, is being held at Fulton County Jail and faces one count of felony murder.

But Sams said that isn’t enough.

“I hope they put him in an isolated room with a metal bed and my mom haunts him until the day that he dies,” she raged.

“That’s how I feel. You took an innocent person, you took someone that’s harmless to you.”

Riders also blamed Saturday’s killing on the recent fare​-relief program that allowed people, including the homeless, to enter MARTA stations and trains for free during construction and the rollout of new fare gates systemwide, leaving some old gates and emergency doors temporarily open. 

While MARTA reportedly required riders to resume paying fares on the same day as the deadly attack, customer Montanio Huff told Fox Atlanta that “more security” is still needed as violence continues to rock the commuter rail.

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 State Capitol last Sunday.

“This was a senseless and heartbreaking loss,” a MARTA ​rep told The Post, adding how thousands of employees, 280 sworn officers and 30 field specialists are tasked with protecting ​the system’s riders daily.

“We mourn with Margaret Swan’s loved ones and extend our deepest sympathies during this incredibly difficult time. We are committed to do everything we can to protect our customers and employees and respond quickly when incidents occur.”

The transit rep said MARTA ​also has 12,000 security cameras across rail stations, buses, and trains, along with a team of undercover officers patrolling stations​, too.

MARTA is ​meanwhile focused on beefing up security ahead of the FIFA World Cup in Atlanta this month. 

“We’re enhancing those efforts for the FIFA World Cup, including more police, increased partnerships with local law enforcement agencies across the region, and additional personnel throughout the system to support customer safety and crowd management,” the ​representative said.

“We will continue investing in the people, technology and partnerships that keep our riders and employees safe.”