Liam Armstrong, the Long Island teen who died while crossing subway tracks on his 18th birthday, is being mourned by his friends and family. The flag outside Smithtown East High School was at half-staff and friends continued to Tweet condolences with the hashtag #staystrongsmithtown. A friend told the Post that Armstrong had enlisted with the Marines, intending to serve after graduation: "He just thought it was honorable and a good decision for him."

The Post reports, "Two Marines yesterday brought flowers and paid their respects at the family’s Nesconset, LI, home."

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Liam Armstrong memorial at West 79th St. subway station (NY1)

Armstrong and friends/lacrosse teammates Dakota Walsh and Ryan Van Duyne left Long Island for NYC at around 2:30 p.m. on Tuesday and had been "hanging out in Central Park for a few hours before heading to the subway on their way down to Greenwich Village," according to DNAinfo. The trio had gotten onto a 1 train at 59th Street, but found themselves on the uptown train. "After realizing their mistake, they got off at the 79th Street stop and misguidedly decided to take a fatal shortcut across the tracks, sources said."

At 6:30 p.m., Armstrong was struck by an uptown 2 train. The train's motorman tried to stop the train, but still hit the teen. One of his friends had made it safely across to the downtown platform while the other was still on the uptown platform. Walsh later Tweeted, "You're my brother. I'm sorry I didn't protect you."

One friend told 1010WINS, "Liam’s a smart kid, it’s just a mistake that took his life... It’s just a shame what happened. He was just a good person," while another said, “It’s crazy right now — can’t believe it. I’ve been friends with him since elementary school. I’m just concerned about his family, younger siblings. I’m just really upset right now.”.”

A psychologist also told WCBS 2 that teens might not be able to process risks, "Their brains are not fully developed. The prefrontal cortex is booming with energy and stray thoughts. Possibly alcohol in the mix, lower, much lower impulse control and you end up taking a lot of really crazy risks and in this case it was a fatal one."

Mayor Bloomberg said, "You can’t stop somebody from running across the tracks, what do you expect them to do? I mean, it’s a tragedy and I don’t know what you say to the parents. A young kid makes a mistake on a dare — all kids when they grow up dare things and do stupid things and we’ve all looked back and said, ‘but for the grace of God, thank goodness the accident didn’t happen.' In this case, the probabilities caught up with this young kid but it’s hard to fault the MTA for this."

A friend who was invited to go into the city but was sleeping when Armstrong texted him (the students are on spring break) was feeling regretful, "I was supposed to be there. I think about that all the time now. I could have maybe talked him out of what he did."