The train was heading from Cremona, in eastern Lombardy, to Milan's Garibaldi station that morning. The condition of 10 in the injured is being reported as critical.
Cause of the crash is still being investigated.
Rescue workers were pictured climbing through the mangled wreckage to reach those trapped inside.More news: Snapchat now allows stories to be read by people without the app
Dozens of emergency service personnel are on the scene.
The injured were rushed to a nearby hospital for medical assistance.
Three people died Wednesday and nearly 100 people were hurt, including seven with life-threatening injuries, as a crowded commuter train derailed in the eastern outskirts of Milan.
One of the carriages jack-knifed across the track, bent nearly at a right angle.More news: New York City Sues Drug Companies Over Opioid Epidemic
A number of rail services in the city were delayed or suspended while emergency services deal with the incident.
Trenord informed passengers at 8 am of the interruption to services due to a "technical problem", sparking sharp criticism on social media.
An eyewitness told La Repubblica newspaper that: "Shortly before Pioltello the train began shaking and we knew there was something wrong".More news: EU Sanctions Venezuela as Almagro Condemns Presidential Elections