I'm afraid that there is no subway leading you to Beijing South Railway Station directly. So, please considering taking the city-bus. At Dongzhimen bus terminal, you could take the city-bus No.106 to arrive the Beijing South Railway Station. But,please note that the bullet train doesn't depart here. Then, you have to take the ferry bus from here to reach the south square of Beijing South Railway Station, where you could catch a bullet train~

A little complicated~ Good luck!

What if I take the airport shuttle bus directly to Beijing Railway Station and take a bus to Beijing South Railway Station? What number bus should I take to reach Beijing South Railway Station?

Thanks alot!