my plane lands at 11.15pm is there any other train / bus I could take from Beijing Capital Airport?

Yes, you can take the night shuttle bus which operates from 21:00 to 4:00 am but no subway is available. The shuttle bus night line will stop at Fangzhuang, Xidan, Wangfujing...However, since it is too late and you may have some luggage, it is suggested to take a taxi.