the upper surfaces of the rails are inclined slightly towards each other, typically on a slope of 1/20, and the rims of the train wheels are angled in the same way ("coning"). this helps guide the vehicles of the train along the track. each wheel also has a flange, which sticks out from one edge all the way around. this makes sure the train does not "derail" (come off the track) and helps guide the train on sharp curves.