Just in case you didn't know, but plenty of work has been made to create a legal cross country/ free trail and dirt jumps in NYC. This has been brought to by the guys at nycmtb.org & climbonline.org . There's going to be an opening celebration on May 19th and XC Race on the 20th.
Here's some pics of what been happening there:
^ photos by ROB BK
^ photos by Juan
^ photos by Yoni