Abstract: A pair of knuckles each including a base element and a pair of transversely spaced longitudinally extending lugs on the base element. The lugs on each knuckle are pivoted in a pair of lugs secured to the frame of one of the leading and trailing equipment units, the two knuckles being positioned on relatively transversely positioned axes. The knuckles are interconnected by a longitudinal pin extending through the base elements, the longitudinal pin being dimensioned to enable longitudinal play between the knuckles, and relative angular movement therebetween.