Here's what Walthers has to say about these -
Stand-Alone Well Cars entered service in the early 1990s as larger and heavier containers came into use. With a truck at each end, they have a larger carrying capacity and are easier to maintain than the articulated cars. Some owners run them connected with drawbars as three, four or five "unit cars." Stand-alone cars are now being used with heavy shipments such as the movement of garbage and some foreign shipments moving in 20' containers.
This detailed car has a die-cast metal body for maximum weight and superb on-track performance. The cars are ready to run and come prepainted in a variety of colorful schemes. And it's easy to model loaded cars, using Walthers 40 and 48' Containers, sold separately.