The homemade device, which was concealed in a bucket within a shopping bag, exploded around 8:20 am on Friday, as the train was carrying commuters from the suburbs, including many school children.
Scenes of panic followed the explosion, Roberta Amuso, a Bergamo native working in London, said.
“I didn't clearly hear the explosion, I didn't even have time to understand,” she recalled.
“I was hit by a mass of people pouring out of the train. I ran towards the exit too. Those who tripped up and fell to the ground were trampled. They were moments of fear.”
The Islamic State group claimed responsibility for the attack, which it said was carried out by an affiliated unit.