The Collaborative International Dictionary of English v.0.48:

Singletree \Sin"gle*tree`\, n. [Cf. Swingletree.]
   The pivoted or swinging bar to which the traces of a
   harnessed horse are fixed; a whiffletree.
   [1913 Webster]

   Note: When two horses draw abreast, a singletree is fixed at
         each end of another crosspiece, called the doubletree.
         [1913 Webster]