A movable subframe or slider box for tractor-trailers includes a pair of elongated, longitudinally-extending, spaced-apart, parallel main members, and a plurality of cross members which extend between and are attached to the main members to form a rigid slider box structure. The slider box includes a retractable pin mechanism for selectively positioning the slider box beneath the trailer. The slider box supports one or more axle/suspension systems which in turn support the vehicle wheels and tires. The slider box is free of hangers for pivotally mounting beams of the axle/suspension systems on the slider box. The beams instead are pivotally mounted directly on the main members. Each main member has an inverted generally U-shaped configuration which forms a channel, so that a bushing assembly end of each beam nests within the channel of its respective main member and is pivotally attached thereto.