Oops didn't see the post. Guessing Jackson Middle is the express track on the White Plains Road Line and the 42nd Street spur is the track between the two express tracks between 34th and 42nd Street which is also the same track that the 3 uses to turn during overnight hours. This definitely doesn't mean the shuttle track because it doesn't have a connection to anything but the northbound local track. There is a center track at 135th and Lenox, but I'm not sure if it's long enough for a full length train to fit on without blocking the other tracks. Obviously it would not be used for transfers to 240th Street Yard, but could possibly be used for transfers in and out of Lenox Ave Yard.