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So, what does keep the train on a track? :D

(inspired by http://xkcd.com/1145/)

#xkcd   #science  
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Joseph Perkins's profile photo
 
Two things. And an additional thing as a backup. One: Gravity for the up and down. Two: The camber of the wheels for side to side. And, just because nothing is perfect, Three: The wheel flanges.
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