Going till Delhi? Intend to travel in sleeper class? No need for booking tickets in advance! S5,S6 and S7 coaches are dereserved for BE-DLI. Just buy sleeper class unreserved tickets (Do mention "Sleeper class" ticket while buying) and enter these coaches. Even if you book in advance you'll get "Reserved" seats in these very "Dereserved" coaches. So no point.