First go to the Imperial College, then the Yonghe Lamasery. After that, take subway line2 at Yonghegong Station, alight at Xizhimen and you’ll find the Beijing North Railway Station is right there. For your reference, the Great Wall has two rush hours recently (10:00 and 14:00-15:00) due to the traffic jam on high speed way, so there may be as many tourists in the afternoon as in the morning.