Originally Posted by jiejie

Is there a reason you must purchase at South Station? If you will be staying in Beijing prior, it is probably easier to purchase at one of the numerous satellite offices all over the city, if you want to do it yourself rather than have hotel do it. www.cnvol.com has a Train Ticket Office Search function on it. Or hotel can direct you to closest one.

Outside of holiday periods, I would think that you could get a ticket on one of these two trains a couple of days in advance of your departure date, particularly if you're not fussy about taking either first or second class.