The CTA suspended Pink Line service from Polk to the Loop and service on the Orange Line is also disrupted. Shortly before 7 a.m., service was resuming.
Police said a 32-year-old man was on a train at about 4:30 a.m. when two male suspects took out a cutting instrument and stabbed the victim in the left side.
The victim and the suspects got off the train at the Washington/Wells station, police said.
The victim was transported to Northwestern Memorial Hospital in serious condition.
"It has been feeling a little sketchier lately," CTA passenger Amanda Raitenbach said. "There have been more incidents of people getting rowdy into fights, smoking on the train."
"I'm really a protector. I watch everything going on. I mind my business first of all, but if I see danger, I'll make sure everybody is good," said Mark John.
Friday morning's stabbing follows a rash of violence involving the CTA, including a community alert issued Thursday, hoping someone can identify suspects from a Red Line platform robbery that happened early Monday in the Loop.
It also comes as Rhanni Davis faces first-degree murder charges after Chicago police said he shot and killed four sleeping passengers on board a CTA Blue Line train in Forest Park earlier this month.
CTA President Dorval Carter addressed crime on the CTA during an appearance at the City Club on Thursday.
"If we want to fix these problems - we have to create an environment in which these opportunities can exist that allow people to choose alternatives than crime," CTA President Dorval Carter said. "Public transportation is one of the ways you do that."
It's still unclear what was happening in the moments leading up to the stabbing. Detectives are still investigating.
No other injuries were reported, police said. Area Three detectives are investigating.
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