The robberies at gunpoint happened early Saturday at the Loyola Red Line station on North Sheridan Road.
Some passengers expressed concerns that the suspect was still on the loose. The CTA is working with Chicago police to investigate the armed robbery.
Red Line regulars who usually board the train at the Loyola station were surprised to learn about an armed robbery that took place on the "el" platform early Saturday morning.
"The most that I see is people asking for money, which technically is illegal, but nobody seems to bother them too much about it," said CTA passenger Alan Salmi.
According to police, a man pulled a sawed-off shotgun out of his backpack around 3 a.m. Saturday and demanded cell phones from other passengers.
Although no one was injured, the suspect apparently got away with a half dozen cell phones.
The CTA is reminding passengers to be aware of their surroundings and to keep cell phones and other electronics out of sight.
"I'm guilty of that myself, but everybody's always on their phone. The phone is the number one source of communication, so people are trying to text and whatever and make the time go by faster and communicate with everybody," said CTA passenger Corsica Campbell. "I wouldn't say it's the best thing, because you can't see if someone is sneaking up on you."
"I always keep my stuff with me and keep it concealed, not out in the open, and if I don't show it, they won't have anything to rob me for," said CTA passenger David Wilson.
"I do try to be mindful, especially with the headphones in, you have to be cautious of your surroundings," said CTA passenger Colin Linke.
Police are reviewing surveillance video of the suspect and are following up on leads.
The Loyola CTA stop, just across the street from Loyola University in Rogers Park, is described as diverse and friendly. Some passengers say it is safe and that they want it to stay that way.
"We need to be paying attention so that we can protect and cover each other in case there is a crime," said CTA passenger T'oni Gray.
According to police, the most common crimes on the CTA are theft-related offenses, although compared to this time last year crime is down about 12 percent.
Police have been patrolling the area around the Loyola stop keeping an eye out for the suspect.