This song is about the September 11, 2001 terrorist attacks on the US, specifically in New York City (turnstiles are part of the subway system). It was the first song that the band ever wrote.
Gerard Way, the band's vocalist, had given up on playing music after being kicked out of his first band as a teen for his lack of guitar skills. By 2001, he had a steady job designing action figures for the Hoboken-based company Fun House, but the 9/11 tragedy forced him to reevaluate his priorities. "I almost used it as a catalyst," he told biographer Tom Bryant (Not the Life It Seems: The True Lives of My Chemical Romance). "I decided to take it, to say f--k off to the rest of my life and start a band. I just knew that’s what I wanted to do right away."
The morning of September 11th, Gerard was aboard a ferry bound for Manhattan when the two planes struck the Twin Towers of the World Trade Center. He recalled to Noisey: "I didn't see the planes hit. I did see the buildings go down, from I'd say fairly close. It was like being in a science fiction film or some kind of disaster film - it was exactly that kind of feeling. You didn't believe it. You felt like you were in Independence Day. It made no sense. Your brain couldn't process it. And for me it was a little different. I'm very empathetic and I'm kind of a conduit emotionally, so I pick up a lot of stuff in that way. There was about three- or four-hundred people around me - and I was right at the edge. All these people behind me, they all had friends and family in those buildings. I didn't. So when that first building went, it was like an A-bomb went off. It was like just this emotion and it made you nauseous."