which is the fastest way from Capital Airport to Beijing South railway station?

Taxi of course is the fastest way. It takes 40-50min and costs CNY110-120.

Alternatively, you can take airport express to get to Dongzhimen Station first. It takes around 30 mins. And the ticket fare is CNY 25 per person. Then take subway line 2 to Xuanwumen Station and line 4 to South Railway Station directly. The duration is about 30 mins. And the ticket fare is CNY 5 per person.

In addition, shuttle bus line 10 can take you there in around 80min. The bus fare is CNY24.