Metra said the employee hadn't been at work since feeling sick a week ago.
All cars currently in use on the line have been cleaned and disinfected, according to Metra. Other employees who had direct contact with the conductor have been notified and are self-isolating.
Three other Metra employees were also diagnosed with the virus. All of them are field workers who had no contact with riders, Metra said.
"We will continue to monitor the situation and work with health officials," Metra said.