You can take a taxi, and it takes about one hour respectively from the airport to the Beijing South Station and Beijing Station. If the flight you take does not delay, it is possible for you to take the train D61 at Beijing South. And it is better for you to arrive at the train station around 16:30. Besides, if you miss a train, the original train ticket will become invalid. Then you need to buy the ticket of the next one and generally there will be no problem as long as the tickets are not under great demand.

You can take the Airport Express first at the Airport Terminal 3 or Terminal 2 and get off at the terminal - Dongzhimen. The Airport Express runs every 4 to 5 minutes and the fare is CNY25. The whole way takes about 20 minutes including 2 to 4 minutes’ waiting. This is faster than taking the cab. If you take the cab at the Terminal 3, you have to wait for some time because there are always many people waiting for cab.

After arriving at Dongzhimen, you need to take Subway Line 2 and get off at Hepingmen Subway Station. The ticket price for subway is CNY2 and it will take about 18 to 20 minutes. Subway Line 2 is a circle line; be sure to take the nearer direction (->Dong Sishitiao Direction). Go out of the Hepingmen Subway Station from the south exit, there you can take a cab directly to reach the South Square of Beijing South Railway Station and the cab fare is around CNY15 (15 to 20 minutes).

If you miss the train D61, you have to buy the ticket for the next train. Your transfer time is a little pressing. Wish you good luck!

Great thanks - in particular for the subway advice!

Do remember to go to the SOUTH Square of Beijing South Station where you can take the train directly. The North Square is under construction and it is several kilometers from the South Square. Transportation between the two squares is rather frustrating.