One of the trains' locomotives derailed in the violent impact on Friday, while another carriage was thrust into the air and came to rest on the other train, television pictures from the scene showed.
Rescue workers and emergency services battled time and fog to get close to the wreckage, strewn on a field between two small towns north of Bologna.
Fire services said about 13 people were killed when the two trains collided. They were travelling on the same track between two stations on the plains north of Bologna.
More than 50 people were injured, at least three of them seriously, police said.
An Italian railway statement said the accident involved an inter-regional passenger train travelling from Verona to Bologna and a freight train. They crashed between the towns of Crevalcore and Camposanto.
Fire brigade officials said the thick fog in the plains at the time of the accident had made it difficult to bring cranes close to the track.