yes, i have seen flatbeds hauling intermodal. could have been that particular driver, owned all his equipment. otherwise, at the ports there are the chassis for the containers.
They use this type of trailer. Its called a container chassis. When they put the container on top of the trailer it locks onto the pins you can see at all 4 corners of the trailer... Some guys pull their own trailer.. Most trucks i think use the ports chassis (im not sure how thats handled im not a can driver)... If you ever come to the pacific northwest you will see these trailera with 4 axles and lift axles on them. They get pretty crazy with how much weight they can haul. You can haul it on a flatbed as well. Like i have . some flatved trailers have container locks on them that do the same thing as the pins on the container chassis. Keeps the flatbedder from having to chain it down and strap it .
Some companies are buying their own chassis. Better to not pull the crap that the terminal companies have. Duncan and sons in Phoenix has their own. Lighter weught, aluminum wheels, good tires , etc.
the 20 footers are the most top heavy . I hauled a couple of 20 footers that had 48k in them , and it was a very sketchy ride .