Well cars are used to ship intermodal containers. Shipping containers themselves can hold a wide variety of goods, so really just about anything could be riding on a well car. A well car, also known as a double-stack car (or also intermodal car/container car), is a type of railroad car specially designed to carry intermodal containers (shipping containers) used in intermodal freight transport. The "well" is a depressed section which sits close to the rails between the wheel trucks of the car, allowing a container to be carried lower than on a traditional flatcar.
Well cars come in a variety of sizes. The size of a well car is based on the size of the containers it carries. The most common are 40 feet, 48 feet and 53 feet. There are also rail cars known as "all-purpose" well cars, which are equipped with hitches that also allow them to carry semi-trailers.