The teenager who died after poking his head out of a moving train is believed to have been part of a group of boys who broke into an unattended rear driver's cabin, wreaking havoc inside.
The boy – 17-year-old Jack Irving, of Beaumaris – was believed to have been with three other teenagers, who broke into the back carriage of the Caulfield-bound train, near Malvern train station on Friday night, Rail, Tram and Bus union Victorian secretary Luba Grigorovitch said. Jack's younger brother, Nicholas, was with him at the time.
It is understood Jack stuck his head out of the moving train, and was clipped by a signal panel before being dragged out of the train, Ms Grigorovitch said. The accident happened about 200 metres from Malvern station.
Ambulance Victoria spokesman Ray Rowe said paramedics treated the teenager at the scene for upper body and leg injuries.
He was taken to The Alfred hospital in a critical condition but died shortly before 11pm. No other passengers were injured.
Ms Grigorovitch said the doors to the drivers' carriages – of which there are two – are tightly sealed, and would have been difficult to pry open.
"It wouldn't have been easy — those things are locked tightly," she said.
"They must have bashed against the door and tried to open it up." .
"This type of behaviour is reckless and dangerous, and people need to understand that if they choose to behave this way, there will be serious consequences."
Tributes for Jack have been posted on social media, with the St Leonard's College teen described as a sporty and lovable guy.
Jack's father, Tim Irving, described his son as a "beautiful boy and big brother" who will always be remembered.
"Our beautiful boy and big brother. In God's presence now and made this world that bit better for 17 years. Thank you for being you Jack. Eternal love, Dad xxxxxxxx".
He wrote that receiving the phone call from hospital was "like a nightmare".
"We can't believe what's happened."
Friends have called the late teen an "angel" who was liked by anyone who met him.
Jack's girlfriend, Elyse Waterman, was told by her parents about Jack's death just as she was preparing to visit Melbourne. Her mother, US-resident Stacey Waterman, wrote about the phone call on Facebook, describing it as "devastating".
"Paul and I had to tell her that the love of her life was gone. I don't think she is ever going to recover from this," she wrote.
Ms Waterman said that Jack would visit the family in New Jersey often, and was "liked by anyone who met him".
"I feel honoured to have had him in our lives," she said.
"I can't believe he is gone - a beautiful 17-year-old boy with his life ahead of him. I don't know how we are going to live with this loss, or more importantly help Elyse get through it. He was so good to Elyse and everyone else. I'm struggling to make sense of this all and having an incredibly hard time."
On Sunday, Elyse posted a picture of herself and Jack cuddling on Twitter, thanking family and friends for their love and support.
"Thank you to everyone who has reached out to my family and me during this time. RIP to the most amazing guy," she wrote.
The couple have been described by friends as "the strongest and happiest couple".
Jack was also a popular member of the Oakleigh Cricket Club.
Acting Sergeant Andrew Kiss said an investigation would determine how the accident happened and whether something was being done illegally.
"It appears he may either been hanging from the side of the train or hanging from the train," he said.
A Victoria Police spokeswoman told Fairfax Media on Monday that the teenager was in a carriage towards the rear of the train.
A Metro Trains spokeswoman said Metro was continuing to support police in their investigation.
Ms Grigorovitch said the union would use the tragic event to call on the state government to launch an education campaign for travelling in public.
"We called on the state government to initiate this campaign before and got no response," she told Fairfax Media.
"At the end of the day, it's their responsibility to make the public understand the risks associated with foolish behaviour."