can you buy one way train ticket in person at guangzhou east station to hong kong station

i would like to know if it is possible to buy an one-way train ticket at guangzhou east station to HK in person; also is the information correct when you purchase a ticket online and your departing station is at guangzhou east station and your arriving train station is at HK that you cannot pick up the ticket at any train station in china? what are my options...thank you kevin