The airport buses from Beijing airport to Tianjin are available in Beijing airport. Why not take it directly? At present, there are no airport buses to go to south railway station. You can take the Airport Line 1 to Fangzhuang. Fangzhuang is about 5 kilometers from the south railway station. Then, hail a taxi to go to the south railway station.

Thank you ms Evelyn. I am looking thru the subway map of beijing..how come I can't see the Fangzhung station. I see that the airport line ends at Dongzhimen Station, right? Please clarify

Oh, I think I have not replied clearly. The Airport Line 1 to Fangzhuang is a bus not a subway train. So, you can not find the Fangzhuang Station on the subway map. I mean you take the Airport Line 1 to get to Fangzhuang. The cost is CNY16. Then, just take a taxi to go to the South Railway Station. Fangzhuang is not far from the South Railway Station. The taxi fee is less than CNY20.

Or you may take the Airport Express (subway) to Dongzhimen.
Airport Express (Line L1): Beijing Capital International Airport (T2, T3)- Dongzhimen
After you arrive at Dongzhimen, just take the Bus No.106 to go to the South Railway Station. Only the Bus No. 106 can take you go to the south railway station from Dongzhimen bus stop directly.

If you adopt the second way, the total transpotation fee will be a little cheaper than the first one.

Thank-you Ms. Evelyn. Your explanation is very clear. By the way, can we take our luggage into trains (normal city line train) and also into bus like Bus 106 to South Railway Station. I know some cities don't allow big luggages into their metro trains or buses.

If your luggage is not too long and too big, it is allowed to take into the subway train and bus.