The semi-trailer is a model that runs forwards for a short distance, then backs up, emitting a "beep" when it does the latter.  I used 0.25" chain for mode of transport since it was plentiful and easy to use.  It's overkill to use two chains but I did that for stability of the semi-trailer sitting on top...and I had lots of chain.  Hover over photo for description.
Video:  SemiTrailer