Abstract: An elongated trailer includes a bed supported on an undercarriage, wherein the undercarriage is shiftable longitudinally between roading and loading positions. At least one beam is provided on the bed and includes a lower flange defining a tracking surface, a web extending upward from the central longitudinal axis of the flange, and forward and rear ramps formed in the tracking surface. The forward and rear ramps are spaced longitudinally from one another and extend laterally inward from opposite sides of the tracking surface. The ramps are of a width greater than one half the width of the tracking surface so that the ramps overlap one another when viewed along the central longitudinal axis, and each ramp extends beneath the web. A pair of rollers are supported on the undercarriage for rotational movement and bear against the central longitudinal axis of the tracking surface and engage the ramps when the undercarriage is shifted to the roading position.