Need help with a name of a book?

theres this book about a girl who lives in NYC, and she meets a boy my mistake who lives underground in the old subway stations, wheres there a whole nother world under the city. I knw the book is called something cl the Upsiders, or the Down siders. I really dont know, but I just need the Off. Name of the book and Author. P.S. this is not for school. :)