Bonus question: Let's say you do In-City Check-In at Kowloon Station and use an Octopus to pay for the Airport Express. It will charge you the AE fare for Kowloon at that point, even though you may not get on the train (you have 24 hours to go through the train turnstiles and have that trip covered by the charge made during in-city check-in). Within 24 hours, you go through the AE train turnstiles at Hong Kong Station. Is it smart enough to charge your Octopus just the difference in fare between Kowloon and Hong Kong trips to the airport? Or do you get away with not paying the extra fare? Or does it not link up your previous fare payment at Kowloon and charge you a full fare?