A woman told police she was standing in front of a kiosk just before 11:10 p.m. at the Harrison station when a man snatched a bag out of her hand.
The bag was holding her wallet and phone, CPD said.
The thief ran off, and the woman was not hurt.
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Area Three detectives are investigating.
CTA-related crime is up 40% this year compared to last.
In March, CTA officials said they would add more security on trains and buses, but they have yet to be fully deployed.