The accident was at Victor Emmanuele II station when one train drove into another waiting at the station, officials said on Tuesday.
Mayor Walter Veltroni, speaking at the scene of the accident close to Rome's central train station, said a 30-year-old woman was killed.
ANSA, the Italian newsagency said the woman was Nigerian. Early reports that two people were killed in the incident were mistaken, officials said.
"It was hell. I saw a ball of fire and I heard a terrible cry," a young woman told the television station Sky TG24.
"I was hit on the shoulder. A man in front of me fell and people trampled on him getting out of the car. I was really lucky," she said.
Giulio Laurenza told the daily La Repubblica's internet site that his wife, 49-year-old Tiziana Guarnieri, "Was in shock" when she called him.
"She told me she fell down along with other people, and when other panicked passengers trod on her she couldn't get up," he said.
Piero Marrazzo, the president of Rome province, said up to 20 of the most seriously injured people had been taken to hospital.
Achille Serra, the city of Rome administrator, said: "It would not be opportune to speculate now on the causes of the accident. A magistrate... will make a judgment following his investigation."
Several dozen people with minor injuries were seated around the entrance to the station awaiting treatment.
An injured man is carried to an
ambulance by a rescue team
"I was in the metro train that was stopped at the station when I saw the other train run into us. I was horribly frightened," Alberto, a passenger, said.
Security services sealed off the area, and police manned the station's four entrances.
About 15 ambulances were despatched to the scene of the accident, which caused major delays on Rome's traffic.
The news television station Sky TG24 showed victims being carried out on stretchers while other passengers emerged looking dazed.