Nowadays they have made a norm to fit the AC coaches at either the start or at the end of the train so that they cannot be accessed by outsiders(Non-AC ticket holders) and vestibule door closed-some trains have AC coaches in middle owing to bifurcation or slip coach servies of trains(Eg: 6222 kaveri express has a 3A coach and then a general followed by S-12-14 and 2 more AC coaches because the last set of coaches are detached at bangalore city-similar is case with Nilgiri too where coaches are detached at Coimbatore Jn.)