How often does the trains go from beijing south Railway directly to the Capital Airport?

If by subway, you need several transfers. Take subway line 4 from the station to Xuanwumen, transfer to line 2 to Dongzhimen. Take airport express there to the airport. The interval of subway is 3 – 10 minutes.
You can also take airport shuttle bus from the south railway station. Line 10 will take you to the airport directly. The bus runs from 07:00 to 19:30 with an interval of not more than 30 minutes.