Is there any airport express after midnight? How am I to get to Beijing Nan Zhan train station?

Well, the last trains are scheduled at 23:10 in T2, 22:50 in T3. So there is no express train for you to take at that time.
The earliest schedules are 06:35 (T2) and 06:20 (T3). Thus you can take shuttle bus Xidan Night Line to get to Civil Aviation Building first. The operating hours of this bus line are from the midnight to the the last domestic flight arrival. The ticket fare is CNY 24 per person. Then take a taxi to the South Railway Station. The cab fare is about CNY 25 at night.

If you still wanna take the airport express in the next early morning, take the earliest one to Dongzhimen Station first. The ticket fare is CNY 25 per person. Then take subway line 2 to Xuanwumen Station and line 4 to the destination. It costs CNY 5 per person.

Thank you very much..It has been very informative n detail.. All very well noted . Tqvm ..