It really was a commute from hell.

December 7 will mark the 20th anniversary of the day crazed gunman Colin Ferguson went on a murderous rampage aboard the Long Island Rail Road, pulling out a 9 mm pistol and opening fire, killing six people and wounding 19 others.

As the 5:33 p.m. train to Hicksville approached the Merillon Avenue Station in Garden City, Ferguson walked down the aisle of the third car and took aim.

For three terrifying minutes — he went back to his seat in the rear of the car to reload — Ferguson calmly blasted away at the 80 panicked passengers.

“There was panic, somebody was saying, ‘This is real, it’s not fireworks,’ ” Kevin Zaleskie says in the “The Long Island Railroad Massacre,” a new documentary. “People started running up the aisle, trying to get out of the car.”

Zaleskie, an analyst at IBM at the time, put his briefcase up to his face, a desperate shield. He wasn’t shot.

“I thought I was going to die right there,” he says, looking spooked to this day.

The aisles, says survivor Tom McDermott, who took a bullet in his left shoulder, were “literally littered with bodies.”

But Ferguson wasn’t done.

“He started to move to my end of the car, gun emptied. He stopped, went back. I saw him bend in the seat, put another clip in the gun and now come down the other end of the car, ” said survivor Robert Giugliano.

“He was getting closer and closer, bullets are flying around and this girl Maria, who is sitting next to me, she starts crawling, and I’m looking at her saying, ‘No, no, don’t go there!”

“Before you know it, she looked up, and he was maybe eight to 10 feet away, and he pointed and shot her, shot her in the head. Her head exploded, right next to me . . . it was horrible.”

The woman was Maria Magtoto, a 30-year-old Westbury lawyer who was slaughtered along with James Gorycki, 51, Amy Federici, 27, Mi Kyung Kim, 27, Dennis McCarthy, 52, and Richard Nettleton, 24.

“At that instance I looked up, and there he was, looking at me staring at me straight in the face,” Giugliano recalled. “I said, ‘Dear God, I’m dead.’”

But he wasn’t.

The 58-year-old Giugliano was shot in the arm and the bullet traveled into his chest, but he survived — the last person shot by Ferguson, who was tackled to the ground by three straphangers after his second 15-round clip went dry.

“He was standing in the aisle with the gun in his hand — and [as] we came toward him, the gun dropped,” hero Kevin Blum told The Post at the time. “He had a blank look — like he knew he had done something wrong.”

Scribbled notes found in Ferguson’s pocket revealed a twisted mind, blaming “racism by caucasians and Uncle Tom Negroes.”

“NYC was spared because of my respect for Mayor David Dinkins and Comm. Raymond Kelly,” one note read.

“Nassau County is the venue,” it adds with the nonchalance of a party planner.

Ferguson, who was declared sane, represented himself in a bizarre murder trial and was convicted of six counts of murder and sentenced to 315 years in jail. Now 55, he’s housed at the Upstate Correctional Facility in Franklin County, a prison for inmates with disciplinary problems.

Five years after the massacre, Ferguson told The Post he was innocent, blaming everything on a “conspiracy.”

“It was the finale of the setup. I was on the train, yes, but I was not responsible for that terrible incident,” he said.

Documentary director/producer Charlie Minn said he made the 90-minute film as a tribute to the resolve of survivors, and to preserve the memory of victims, one of whom, Kim, he knew from Herricks High School.

“I have that Long Island pride,” he said.

Victims’ families said their grief remains.

“There is never closure — I hate that word. It will never bring closure. How can it bring closure?” explains Carolyn McCarthy, who was transformed by her husband Dennis’ murder from a suburban housewife to a gun-control advocate who was elected to Congress in 1996.

Lisa Combatti — who was seven months pregnant when Ferguson shot her in the hip area — said she rides the 5:33 train every anniversary and sits in the same seat.

“It’s my act of defiance,” said Combatti, whose 19-year-old daughter, Kimberly, now attends Bucknell University.

Amy Federici’s parents donated their daughter’s organs to four different people, all of whom are now dead — save for Betty Janko, a Texas woman who lives on with her kidney.

“I’m not sure I would have lasted this long without her,” Janko, 70, told The Post, adding that she still keeps in touch with Amy’s parents, Jacob and Arlene Locicero.

When victims gave their statements at Ferguson’s sentencing, Giugliano, a Coney Island native, famously threatened to give the killer a Brooklyn-style beat down.

“Five minutes. That’s all I need with you. Five minutes,” he growled.

He still feels the same.

“Forgive? That’s not in my vocabulary,” Giugliano said. “If I have that one chance, I’m taking it.”

But today, the Howard Beach resident actually celebrates Dec. 7 — the day his granddaughter, Savina Dattolo, was born two years ago.

“The day is now overshadowed by her,” he said.