From CWB Chicago:

A 29-year-old man told police the offender lunged at him as he sat on a Blue Line train minutes earlier. The offender began punching the victim in the head and face, then took unspecified property from the man before exiting the train car as it stopped at LaSalle, police said.
A private citizen intercepted the offender on the platform and held him at gunpoint until police arrived, a source confirmed.