The newly-turned 18-year-old was hit and killed by a northbound No. 2 train at West 79th Street on the Upper West Side around 6:30 p.m. Tuesday.
Liam Armstrong was a student at Smithtown High School East, where messages of sympathy are pouring in as classmates react to his sudden death.
Armstrong reportedly crossed the tracks after he and his friends realized they were on the wrong side, and because his friends dared him. One friend apparently made it across. Armstrong did not.
Police say Armstrong and three friends were in the city for his birthday celebration and were trying to head downtown when they realized they were on the uptown side.
Hundreds of people were on the train and the platform when it all happened.
"It was a sudden stop, it was rougher than usual," rider Esteban Conde said. "People going forward, trying to hold on."
Riders were escorted from the express track and onto a No. 1 train to get to the platform. Amanda Santana was horrified by what she found.
"They evacuated us, and there were like body parts under the train," she said. "I've never seen anything like that."
Police sources say Armstrong had a bottle of rum in his backpack.
"It's sad," one rider said. "Now his parents have to get that call no parent wants to get."
Friends and family members are sharing their remembrances on Twitter using #StayStrongSmithtown. A vigil was held for the teen Wednesday night on Long Island.