It does look from the pictures that I have seen of the Atlas car, though, that it is a lot smaller than either the UTLX or GATX car. For example the Atlas tank does not have [unlike me
] a saggy pot belly that extends down below the bolster line, between the trucks. Until I can get hold of one of the Atlas bodies, it is difficult to see what the possibilities are, but it does look as if the body is significantly smaller than the 'jumbo' or 'whale cars.
I would love to hear differently!

Are you looking at the right Atlas car? What you're describing sounds like the 33,000 gallon "jumbo". There is confusion since there are some info sources referring to the "jumbo" as "whale" or "whale body". The hobby community, at least in my experience, is usually talking about the 60,000 gallon, 4-truck 94' car when "whale" is mentioned.
Here's Mark's (Spookshow) page for the "94 foot tank car":
http://www.spookshow.net/freight/atlasroco94tank.html . Body definitely extends down well below bolster level.