There is a way to transit in CCU. You have to go down to baggage claim and find that one elevator that will take you up to departure security. This is guarded by a CISF jawan who will check things over and over trying to match you with one of her/his lists. You are better off going out normally and queing with the regular lines.

Domestic transit requires rescreening for now. The authorities are working on a domestic transit concept in the major hubs to find a way to eliminate this though.