We will be in HK from Oct 15 to 18 and will be taking the bus from HKIA to Hung Hom Railway Station.

Yes, you can buy the Airport Express Travel Pass with one single journey for HKD 250. When you return to HKIA for your onward flight, you may use it to ride Airport Express train. You can buy it at the round counter upon arrival. Also, you can use it to ride on MTR (except East Rail Line First Class, Lo Wu and Lok Ma Chau stations), Light Rail and MTR Bus (Northwest New Territories) for 3 consecutive days.

After buying it, you can top it up immediately, and use it as a normal Octopus Card to ride on buses, ferries, taxis, trams, etc. Before leaving HK, you can get the balance at any MTR/ Airport Express Customer Service Center, and a refund of HKD 50.

It will be very convenient for you! Have a try! :)