Preloaded trailers are used in some places to prevent the tractor unit from waiting for a new load of logs to be loaded. Below you see a harack type of log trailer which can be loaded and left to stand at a landing or along side a road. When the tractor unit and the empty trailer it is hauling return to the woods, the driver merely switches trailers. The returned, empty traile ris then reloaded and set out again to wait for the tractor.