The fast trains are in Beijing South Railway Station. It may cost one hour or more to the station from PEK. You can buy the ticket on the same day and the bus fare is 58/69 Yuan. Take express train and subway from airport to the SOuth Railway Station from PEK, the route is like this: express train from PEK to DOngzhimen, 25 yuan for ticket fare; Subway Line 2 from Dongzhimen to Xuanwumen then change to Line 4 to Beijing South Railway Station, totally 2yuan. My suggestion is the fast train may be very busy in holidays and weekend. If possible, you can buy the return ticket at Beijing South Railway Station.