It make sense, anyway. Other option, take airport bus line 2 to Xidan then change to subway Line 1 to Dongdan. A bit detour anyway, but simpler. When you get to Beijing Railway Station by airport Line 3, you can get out to take bus 103 or 104 to Dongdan directly. No difficult to spot at Dongzhimen. If you want to go to Dongzhimen first, you can take airport express train from the airport. Dongzhimen is the terminal stop. Then you can change to Subway Line 2 to Jianguomen, then change to Line 1 to Dongdan.