There is only one train from Ximen to Beijing West Railway station. The total journey will cost 33 hours and the ticket price is about 450rmb for hard sleeper and about 700rmb for soft sleeper. The cheapest airport ticket is no more than 1100 rmb, 2.5 hours. You may try to dind much cheaper air tickets at local ticket boiking office. For the time is so long, personally, airplane is recommend.