Two of the victims have been identified by the authorities as Simeon Bihesi 28, and Adrian Collins, 60.
The Cook County Medical Examiner’s Office later said one of the other two victims was a 64-year-old woman named Margaret Miller. The fourth victim has been identified as a man but his name has not yet been released.
Deputy Chief for the Forest Park Police Department Christopher Chin said the shooting appeared to be a “random act of violence”, saying the gunman “shot and killed four victims when literally they were sleeping on the train”.
“This wasn’t a robbery. It didn’t appear that he was in a fight with anybody else,” he said.
On Tuesday, prosecutor Kim Foxx described the shooting as “inexplicable” as she announced the charges.
“It is horrific,” she said. “We want answers.”
Police said surveillance footage showed the attacker walking through the train, and shooting four passengers who appeared to be asleep in two different carriages. The four victims were not sitting together.
The suspect is due to appear in court on Wednesday.
The Blue Line train runs 24 hours a day between Forest Park and Chicago’s O’Hare International Airport and is run by the Chicago Transit Authority, the United States’ second largest public transportation system.
Gun violence is common in the United States, where there are more firearms than people.
Earlier on Tuesday, police in Louisiana arrested an 11-year-old boy who is accused of fatally shooting the city’s former mayor and his daughter.
