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.
Video:  SemiTrailer

Model sits on block of wood, attached to chain below

This is long shot showing the warehouse in the back

It was just a Styrofoam base before

Chopped out the Styrofoam

Motor that drives the chain. Switch reverses the motor

Close-up of the "used" drive motor

Photos of two surplus switches one to move forward, other to move tractor backwards

Mockup on the test bench. Complete, compact, package shown in next photo

I usually make proper CAD drawings...too lazy this time
Parts list:
H-Bridge motor control
5 volt dual relay board for Arduino
555 timer board
forward microswitch
reverse microswitch
DC gearmotor
dual mode LED (R/Gn)
2 lengths of 1/4" chain
Four 1/4 inch sprockets, 1-1/2 inch diameter