People tend to check in during these times:
How to get a seat:: make sure to stand right next to the glass doors. When the other passengers are getting off via the opposite door, lock eyes with an empty seat, doors open, GO!
Beware! Last train to the city is 1030pm! After that it's the buses until midnight, then taxis only. Good luck!
Check this Terminals at the train's doors.
The best way to get between the Beijing Capital Airport and the city. Costs just 25 RMB & 24 mins & stops @ Sanyuaniao and Dongzhimen where you can connect easily to Beijing's extensive subway system.