Right now central Beijing only has two lines, the circle (2) line and the really long red line. From XiZhiMen there's an elevated subway (line 13) which follows relatively sparsely used train tracks (they get a few passenger trains a day, it seems) up to WuDaoKou (where Beijing and Qinghua universities are, and not far from the summer palace) up around looping back to the blue line at DongZhiMen (hence it's name, the DongZhiMen Cityrail). If you travel on the 1 or 2 lines, it's 3 kuai, and if you want to transfer to the DongZhiMen line it's 5 kuai). FYI, though, the transfers are a little less than seamless between the 13 and the 2 because the 13 looks and feels like a subway, but doesn't quite go the whole nine yards at interfacing well with the subway (for example, like the Blue line does in Los Angeles at Metro Center station or MUNI in San Francisco when it comes to BART tranfers.) I'll give more specifics later, with pictures to illustrate.
EAP China! Robert Klein