From Beijing international airport to South railway station

Am traveling Seattle to Nanjing in late May. Have been to the international airport several times, but never have seen signs for subway/trains in international terminal. I need to get to the South Railway station. Is the subway/intercity light rail accessed from the side of the airport that has the Air China hub? Does the light rail go directly to the S train station? If not, where do I transfer to the 4 or 14 subway lines?