I will land to Beijing Capital Airport on Thursday, June 29 at about 16:50 pm.

Well, you can take the Metro Express Airport Line at T3 to Dongzhimen Station. Then you may transfer metro line 2 to the Main Railway Station, Exit D directly. The whole trip will take around 1 hour. However, you need to make sure that you can go through the immigration procedures as soon as possible. Otherwise, you are advised to reserve another train ticket.