1 Only in Tube stations, not train stations (sometimes tube stations and train stations are next to each other, e.g., London Paddington)2 Yes--theoretically. Some places ticket officers may say, sorry, I cannot refund you now---either because of machine problem, or because they are bad3 The barrier will not open if you do not have enough credits in your card. You may be fined in this case.4 Not quite sure--- but the price will not differ for this journey.