Speaking to reporters at the scene, Milan police commissioner Marcello Cardona confirmed that "a material failure was pinpointed" and an investigation for culpable disaster was launched.
The accident happened at about 6am local time as the train travelled from Cremona to Milan's Garibaldi station.
Rescue crews gingerly climbed through them, helping the passengers to escape.
The Trenord train derailed near Pioltello Limito station, under 13 miles outside of Milan, at a switch track.
On social media Thursday, it was ridiculed for tweets blaming delays into Milan on a "technical inconvenience" involving a train.
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Dozens of people were likewise injured less extremely.
Television footage showed fire brigades trying to free people trapped inside the wagons of the train, Reuters reported.
Three helicopters were deployed on the accident site and one helicopter was patrolling the area to immediately bring the injured to the hospital.
An investigation was opened nearly immediately after the crash.
In 2009, 32 people were killed when a freight train carrying liquefied petroleum gas derailed and exploded in Tuscany.
The cause of the accident in Segret, suburb of Milan, is not yet known.