I don't know the details, but I don't think they are all quite the same.
The early Atlas car which is the same body I think as the new Trainman car is a PRR prototype.
the Lionel PS-5 is also a 52 car I think, but a much more modern prototype and features drop ends. It has the more detail than the other I think.
Lionel also makes a 40 steel gon that I think is the same as the old AHM car from the 80s.
I have seen the MTH gons and I think they are a variation of the Atlas car. I don't think they are identical.
the Weaver War Emergency gon is a WWII era (relatively modern) gon built with a steel frame and wood planking sides. This one also has a drop end that works.
Atlas also has a 40 USRA gon that is WW I vintage steel frame wood plank sides. This is a very detailed car that Red Caboose originally developed the tooling for. You can still find the original Red Caboose kits at shows.
It all depends on how accurate you want your model to be. If you are freelancing, then any one is fine pick which you like best.
