Quote - In my final train I might have 3 carriages.  Can I alter the internal carriage BODY Name from   BODY:1 to a unique name so that each of them can have compartments and not be confused?

Yes, you can change the internal name "BODY" to something else. It is usually best to leave the display name, the name that appears in the "name" line, as "BODY", because this will allow you to select it via "Body" in the selection drop-down, other wise it will be placed in the "Body Parts" menu.

I find that using "smartparent" in a figure can cause problems if you try to delete either figure whilst the parenting is still in place. You should change the parent to UNIVERSE before deleting either figure. You should only parent the BODY of the child figure, but it can be parented to any part of the parent figure. A part directly under the BODY of the parent figure, "hip" in a human figure, is usually the most appropriate part, but BODY, or any other part can be used as the parent if desired.