What type of car is designed to carry a trailer or containers, allowing stacking during transport?

The type of car designed to carry a trailer or containers and allowing stacking during transport is indeed the double stack car. This specialized rail car is engineered to accommodate the loading of intermodal containers stacked one on top of another, maximizing efficiency in cargo transportation. The design allows for two layers of containers to be carried, thereby optimizing space and reducing transportation costs per container.

Flatcars, while capable of carrying containers, do not have the specific design features necessary for stacking. They are primarily used for transporting large, heavy items or trailers at a single level. Boxcars are enclosed rail cars used primarily for transporting packaged goods, not containers, and they lack the ability to facilitate stacking. A caboose is a type of rail car used at the end of a freight train for crew accommodation and does not serve the function of carrying cargo containers or trailers.

Hence, the double stack car's design and purpose are uniquely suited to transport stacked containers safely and efficiently, which is why it is the correct choice.