Can I book a train ticket from Hong Kong to Guangzhou online?

Check the ticketing information on the website of the service provider of the Guangzhou-Hong Kong Through train:

http://www.it3.mtr.com.hk/b2c/frmIndex.asp?strLang=Eng

From Guangzhou to  Hong Kong, you cannot book your ticket online , You need to buy at the Guangzhou East Railway Station or you can buy train tickets at your hotel concierge or travel agent office with a small fee is added。