i first discovered this fast track line at the end of february. i was allowed to use the fast track lane together with my companion (travelling in y) upon showing my stc gold card.
everytime i used the fast track line since then (fortunately i was booked on C) there were always some passengers travelling "only" in y but being very late for boarding (tight connection - late arrival). they heavily insisted to stay in the fasttrack lane and finally they stayed :-)

CHris