I've only ever used the FastTrack channel at JFK! All bar one occasion we've been flying in W or Y but with one or both of us having shiny plastic. Once, when we both happened to have shiny plastic, we even took a guest each through the FastTrack - it was empty at the time - and no-one questioned that. We had just checked-in at the Club desk and just wandered over to FastTrack as it was closer than the main security line. Sadly, I don't recall if there was a sign stating who was entitled to use it. Unlike at SEA airport where OW Emerald/Sapphire are explicitly mentioned as being allowed to use the fast lane through security. Won't be flying through JFK T7 for a while now (only internal flights from other terminals) so won't be able to check! I just hope it was someone being overly zealous (and wrong!)!!!