If there is not much, you can take the subway directly to Xizhimen, then take the express train 105, 106 or 205, and get off at Minzu University of China Station (the main entrance) or Weigongcun Station (the small east gate).

It is inconvenient to take the subway from east to west. You can turn left when you get out of Beijing Railway Station. There is an overpass at the subway entrance (there is an elevator going up and down). Walk a few steps to the left after crossing the road, and take the tram 103 to the zoo (there are usually seats from the starting point to the end point). Then turn left, take the underground passage, take the bus across the street, wait for bus No.4, No.6, No.332, 105, 106, and get off at Minzu University of China or Weigongcun Station.