Though I don't have an answer to your question but debatable question is should it always be next to AC coaches? I don't see that being followed by many trains like Lalbagh, Brinda, Sanga, Navjeevan , Pinaakini, Ganga Kaveri Kovai, Pallavan/Vaigai SF. So I guess it makes more sense to have PC in middle of the train rather than always next to AC coaches so that entire train is at equal reach both sides but I am not sure if there is any rule that says PC should always be next to the AC coaches - please let me know if there is any such rule