No, Train doesn't add any extra coach. Passengers who cancel their own tickets at last moment due to sickness or any unavoidable problem help other passengers having RAC ticket to get confirmed. There is also travel agents who book a lot of seats in advance in order to create scarcity in available tickets in general. Some travellers book from them, but one hour before chart preparation of reservation ticket. The travel agents happen to cancel the left over tickets which are not bought from them for that particular train, this helps RAC tickets to get confirmed. So your ticket will surely get confirmed.