The homeless lunatic accused of viciously stabbing great-grandmother Margaret Swan to death during a random attack on an Atlanta train could face the death penalty after being hit with a new federal charge.

John Elijah Matthews, 25, is now facing one federal count of committing an act of violence causing death on a mass transportation system after allegedly slashing the 66-year-old victim’s throat and then stabbing her nearly two dozen times on the Metropolitan Atlanta Rapid Transit Authority Saturday morning.

Matthews, from Decatur, Georgia, faces life behind bars or the death penalty if convicted of the new charge, which was announced late Tuesday.

It piles on to the felony murder charge already hanging over him in Fulton County.

“Margaret Swan was a beloved great-grandmother who was brutally killed in an unprovoked act of senseless violence,” US Attorney Theodore S. Hertzberg said in a statement.

“Her loss deeply affects her family, friends, and our entire community. We will not tolerate murder and mayhem on MARTA, and anyone who uses a dangerous weapon to seriously injure or kill on public transportation may face swift and unrelenting federal prosecution.”

Horrifying surveillance footage captured Swan sitting alone on an Oakland City Station-bound train and looking at her phone around 11:25 a.m. when Matthews suddenly appeared beside her, pulled a folding knife out of his pocket, and stabbed her 20 times in the chest and neck, according to federal officials.

The terrified victim — a mother of three, grandmother of five and great-grandmother of four — desperately tried to fight back as panicked riders scrambled off the train and called for help.

“She was screaming for help, and nobody was helping her,” Swan’s devastated daughter Shanae Sams told The Post on Monday, ripping local officials over the temporary decision to allow free access to MARTA during systemwide renovations for the FIFA World Cup.

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

At least one witness told investigators Swan and Matthews, reportedly homeless, had no interaction before the heinous killing.

The accused slasher allegedly left the Atlanta woman dead in a pool of blood on the train floor before MARTA cops quickly nabbed him on the Oakland City Station, still clutching the bloody knife, officials said.

“Margaret Swan was an innocent woman simply trying to get to her destination. My hope is these charges will bring justice for her and her family,” said Marlo Graham, Special Agent in Charge of FBI Atlanta.

“This crime also impacts everyone who relies on mass transportation to move about their daily lives. Using Atlanta’s train system should be safe and free of violence.”

Matthews is currently locked up at Fulton County Jail.