A bulk material hauling tractor and trailer unit comprises a truck tractor having a bottom dump hopper mounted on its chassis. The tractor tows a double axle pup trailer by an elongated tongue having a span of at least twice the trailer wheelbase. The trailer has nonsteerable axle assemblies and supports a bottom dump material transport hopper. The tractor and trailer hoppers are fabricated of conventional steel plate and structural sections and are formed with a reentrant top wall portion to provide a reduced top opening and increased hopper rigidity. The hoppers present relatively low frontal area and aerodynamic drag characteristics and low center for gravity for exceptional stability for over-the-highway as well as off road hauling.