Thank you ... ALL of you, for your kind words of praise.
'jitte', you asked if "...that little set called a pig?".
If you mean this particular section of the combo :
in the trucking industry, it is known as a 'con-gear' (connector-gear).
For this project (to tie its name in with the names of the other sections)
I'm calling it TRANZ-LINK ... tandem-link for the trailers.
Its wheel-set would be identical to all those used throughout the truck.
All would be 'super-singles' wide-body low-profile Kevlar-sleeved rollers.