Atomic Systems

I just spent an interesting and frustrating few days trying to find out why one of my models kept generating a "Can't load car xxxx into world UID 200000" or some such. I thought I might have had a corrupted part or texture. I tried leaving things out in hopes of finding the offending part to no avail. Finally I thought to break the model down into smaller groups in hopes of isolating the problem. Suddenly the model loaded fine. I had an idea. Then I added the FreightAnim with all the little addons and suddenly no go again. So I decided to Boolean combine a large group of little parts to get the number of objects down to a more reasonable number and now the model loads just fine.
To make a long story short, there seems to be an upper limit to how many objects can be in a group. Does anyone know what that might be? And knowing this now I wonder how much should parts be combined? How does that help or hurt how the model performs? How many groups is reasonable?
I've never had a particular set of rules for grouping things but I can see where it might be necessary. I just don't know the specifics of what MSTS will or won't tolerate.

Not so much objects per group as polys per group.
In the S file, similar textured objects in a group are combined into primitives so the object count is not relevant.

You often can't have more than about 3000-4000 polys in a single group - I'm not sure if this is a total per group or per primitive which are similar textured objects in the same group (matrix)

You can see the individual primitive poly counts in the shape viewer hierarchy view.

Freight anims with groups do not work, so you did the right thing by accident. Moving all the objects into the main freight anim group would have also worked.