From HKIA to Causeway bay, where to buy the train tickets. from airport train station buy?

Hi, the best way for the journey is to take airport shuttle bus A11. You can use the Octopus Card or cash upon your boarding on the bus.
The train? Do you mean Airport Express Train? Well, the ticket purchasing is in the same way. Kindly suggestion that the train cannot take you to the destination. After getting off, you need walk to Central MTR Station to take Island Line to your destination.