A lowboy is a semi-trailer with two drops in deck height: one right after the gooseneck and one right before the wheels. Wikipedia

A semi-trailer with two drops in deck height Wikipedia Disambiguation

A semi-trailer designed for hauling vehicles and other mobile equipment, with two drops in deck height: one right after the gooseneck and one right before the wheels; so named because the trailer's main deck is situated close to the ground for easy loading and unloading of vehicles and equipment. Wiktionary