A number of other people suffered less severe injuries in the derailment.
The accident occurred around 3 pm in the village of Schodnia, when a Pendolino intercity train carrying some 300 passengers hit the vehicle, Pawel Fratczak, the spokesman for national firefighters, told The Associated Press.
The train partially derailed but remained upright. The truck driver was not hurt, he said.
Regional police spokeswoman Monika Mrugala said seven of those injured were in life-threatening condition. Some of the injured were airlifted to a hospital, others were taken by ambulances.
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Mrugala said the truck was transporting another truck on its trailer when it was hit at the crossing, which has no barriers, only warning lights. The truck was badly damaged and its make could not be immediately determined, she said.
Police were investigating the cause of the accident, she said.
The railway tracks need repair and traffic there will probably not be restored before Thursday, according to PKP spokesman Cezary Nowak.
Italian-made Pendolino trains operating in Poland have a top speed of 140 kilometers per hour (87 mph).