It's a specific type of well car that they use to do that, called an "all purpose" car. Typical well cars are open on the bottom, or only have thin straps of metal. The reason is simple, they don't really have to support the container, since it rests on the locking pins at each end of it. Everything in between isn't structural, so it's thin gauge metal. However the problem is that these cars are very special duty. Meaning that they require a specific type of load and can't work with anything else.
Couple years back, Gunderson started tossing around the idea of making these cars more versatile, and came up with the plan of adding a more substantial bottom to the cars, and a locking point (not so much a fifth wheel) at one end. This would allow them to carry truck trailers when containers were not available. These cars aren't overly common though, since railroads still have flatcars they can use. However the versatile nature of them is making these cars more and more popular.
The following blueprint shows how there's literally nothing in the bottom of a well car. This is your average container well type.
