Smartparent DOES work.  I posted, but the post is apparently lost.

In the compartment figure I changed

Parent:  UNIVERSE
 to
smartparent:  Carriage:1

Now when I load the Carriage, then load the compartment from the library, it sticks to the Carriage.  Thanks ChrisCox!

One thing weird about it:  BODY_1 is added to the Carriage's Body Parts list.