A "piggyback trailer" is a flatbed trailer that is designed to be hauled on top of a semi-trailer or rail car. This type of trailer is commonly used in freight transportation and logistics. There are several synonyms for "piggyback trailer", including intermodal trailer, container trailer, and double-stack trailer. The term "intermodal trailer" refers to a container that can be used for multiple modes of transportation, such as rail, truck, and ship. A "container trailer" is a trailer designed to carry shipping containers, while a "double-stack trailer" is a trailer that can carry two layers of containers, increasing the cargo capacity. All of these synonyms refer to a versatile and efficient trailer system that is essential for modern transportation and global trade.