1) Bar on a wagon 2) Wagon bar 
• (n.) The pivoted or swinging bar to which the traces of a harnessed horse are fixed; a whiffletree.
Sin'gle·tree` noun [ Confer Swingletree .] The pivoted or swinging bar to which the traces of a harnessed horse are fixed; a whiffletree. » When two horses draw abreast, a singletree is fixed at each end of another crosspiece, called the doubletree .