The Yellow Line runs from Chicago's far North Side to suburban Skokie.
Chopper 7 was over the scene at about 7 a.m., as crews worked on track conditions near Hamlin.
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It's not clear what caused the derailment.
There were no reports of injuries.
Shuttle buses were available to connect riders from Howard to Dempster-Skokie in both directions, as service was suspended.