The trailer comprises a collector which in the loading phase lifts the bundles on by one from the ground up to a position where they are aligned are behind the other. The alignment is achieved by at least two conveyor belts placed one behind the other. In the loading phase the conveyor nearest to the collector is powdered whilst the other is left free but in the unloading phase both conveyors are powered. When aligned the bundles are pushed onto a loading platform positioned inside the trailer. At the start of loading all the loading platforms are stacked t the bottom of the trailer and as each one is filled it is raised towards the top of the trailer.