I find that in many trains from Howrah/Sealdah like Mithila Exp, Gangasagar Exp, Amritsar Mail, Purbanchal Exp, there are a number of berths available (70-100 in SL and 20-30 in 3A), even for today, tomorrow and EVERY day, even though these trains are heavily booked (WL > 100-200) for their regular long distance destinations. What are these seats? Can I really book these seats, say from SDAH to ASN in 13185 Gangasagar Exp? Thanks