Originally Posted by
HkCaGu
Recent posts and wiki updates report that flyintoCAN-traintoHK is now allowed (but not reverse direction), but it remains unclear how foreign check-in agents will be able to understand and how do you buy bus/train tickets far in advance anyway. Also not every port-of-exit is allowed, notably missing is the high speed train where you exit China in Kowloon.
https://www.it3.mtr.com.hk/b2c/frmIndex.asp?strLang=Eng
The Through Train from Guangzhou Dong to HK Hung Hom is eligible for the 144h twov.
You can buy etickets online.