A high school student was struck and killed by an express train on the Upper West Side as he tried to cross the tracks on a dare.

Liam Armstrong, a student at Smithtown HS East, Long Island, was in Manhattan celebrating his 18th birthday when the accident took place. The popular teen was attempting to cross all four train tracks when he was hit by a northbound express 2 train, at 79th Street.

Armstrong and his friend had mistakenly boarded a northbound 1 train, trying to get from Central Park to Greenwich village. A police source told the New York Post that rather than cross the tracks above ground they dared each other to run across the tracks.

One of his friends succeeded, the other waited on their original platform. Armstrong was stuck by the northbound express train just before 6.30pm.

His leg was severed and he died of severe head trauma.

Police found a liter of Bacardi rum in his backpack and two fake IDs in his wallet.

The train operator attempted to stop the train by pulling the emergency brake but could not do it in time. The power to the train was cut and passengers were held in the train for over an hour while investigators went to work.

A police officer told DNAInfo it was “an unfortunate incident”

He added “You have some kids that don’t know how to come upstairs and go to the other side (of the station). That’s all I can say about this.”

Armstrong’s family in Nesconset, Long Island, declined to comment on the tragedy however friends and the public took to Facebook and Twitter in an outpouring of grief. Armstrong’s private Twitter handle bears the tagline “yolo”, which stands for “You Only Live Once”.

One of his friends present when he died, Dakota Walsh, tweeted “Your [sic] my brother, I’m sorry I didn’t protect you.”

Pal Dylan Nolan tweeted "i love you man...Why my best friend why.

"Ive never cried this much in my life.(sic)"

A friend Kerin Grisanti told the New York Post “I can’t believe this happened.

“He has three little siblings and he loves them — he’s a great big brother.”

Ali Grimaldi, a senior at Smithtown HS West said “He was a very nice kid, always making everybody laugh.

“It was his 18th birthday today, and I think they just wanted to have a little bit of fun.”

Armstrong who was set to graduate this year had considered following in his father’s footsteps and joining the Nassau County police force.

School superintendent Anthony J. Annunziato released a statement saying “[He] was a popular student among his peers and the high school staff.

"He was a vibrant young man who will be missed by all who knew him. Our heartfelt thoughts and prayers go out to his family."

18-year-old Long Island teen Liam Armstrong killed attempting to cross New York subway tracksFacebook