The K-Line well cars are designed to hold a scale 40' container in the bottom position. The Lionel aluminum series of husky stacks is marked to be a 48' well, although they literally are a scale 46'- Lionel, for some reason, made their containers a 1/2" shorter than a scale 48' container. MTH has a husky stack well car designed to hold up to a 48' container in the lower position. K-Line's containers are all 40', although they do make a 20' ISO tank.
Atlas O has the Gunderson 5-car articulated sets, designed to hold a 40' in the lower position, and up to a 45' in the upper position. MTH has a similar car and theirs will hold up to a 48' container in the upper position.
Hope this helps-