The 61-year-old said he was riding the Red Line train northbound when a group of six to 10 people, possibly teenagers, started harassing him, police said.
The man got off the train around 11:40 p.m. at the State/Lake station at 200 N. State Street and the group followed him, police said.
The man said two men shoved him into a group of women and then all the suspects started punching him in the face and body, police said.
A 21-year-old passerby tried to intervene but then the group turned their fists on him.
"That person who jumped in was awesome for stepping in and helping," said Brandon McNews, Red Line rider.
The suspects took the older man's bag and a cellphone from the younger man, police said.
Both victims were transported in good condition to Northwestern Memorial Hospital with facial abrasions, police said.
Earlier that day, a 37-year-old man who did not want to share his name was also attacked on the CTA Red Line. The man said two people robbed him, including a 14-year-old boy.
Police said they responded to the 600-block of South State Street at about 3:30 a.m. The man said the two beat him when he resisted the robbery, and took his possessions. While officers were interviewing the victim, a bystander approached them with cell phone video of the robbery. While they were watching that video, another bystander approached and told police the 14-year-old suspect was still on the train.
Police arrested the teenager, who is charged with one felony count of robbery. The second suspect has not been located or identified, police said.
Many CTA riders had already heard the story by the time they took their morning commute.
"It makes me angry because again, I take these trains back and forth to work," said Red Line rider Kenyetta Henry. "I just wanna go to work and make it home to my kids."
"People say it's Chicago, but sometimes you're not promised tomorrow and it's scary," said Red Line rider Flora Pagan.
Meanwhile, police say they're rushing the processing of the digital evidence in this case, pulling video and any images from all relevant high definition cameras.
Chicago Police Department numbers show there were 1,312 crimes reported on the Red Line alone this year, a nearly 12 percent increase from 2018.
On the other CTA lines, in total reported crimes dropped about 4 percent.
According to data from CPD, theft was the most common crime across all CTA lines, accounting for 39 percent of all reported crime. The second most common was battery.
No one is in custody. Area Central detectives are investigating.