Rescuers are searching the wreckage for survivors and dozens of emergency service personnel are on the scene, reports said. Photographs show three carriages sprawled across the tracks, one of them tilted to the side, smashed into a pole.
Search and rescue efforts ended around 11 am local time and about 100 passengers who suffered minor injuries were given shelter in a nearby gymnasium.
It is not yet clear what caused the derailment.
All passengers were now accounted for, the Red Cross in Italy said.
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The cause of the incident remains unknown but two cars appear to have peeled off the rails and come to rest at an angle.
Confirming the number of dead, an official of the fire department said that the train was completely filled with passengers.
It took about three hours to extract survivors and victims from the mangled carriages, according to the ANSA news agency. Then it braked suddenly and derailed.
The line links Milan and the northern city of Cremona.
Trenord confirmed on its Twitter account that services between Treviglio and Milan had been suspended "due to a technical fault".