HI, Ms.Gibson, the bullet train to Tianjin leaves from Beijing South Railway Station. You could take airport shuttle bus No. 2 to reach Xidan firstly, then take subway line 4 from Xidan Station to Beijing South Railway Station.
Otherwise, you may take the airport express directly to reach Dongzhimen Station, transfer to subway line 2, and then change to subway line 4 at Xuanwumen Station, get off at Beijing South railway station (Beijing Nan Zhan).