I traveled in sleeper class, it was clean but food was really bad in train. So always take home food. Travel hour will take 26 to 27 hours.
In Kolkata great places are there to visit. Victoria memorial hall, exide, birla mandir, kalighat etc. Kolkata is known for food and sweet, my fav puchka (panipuri). So great place for travel. I recommend all to visit.