Bob - Burlington Route: This is the reference to a truck assy. i made earlier --
"I just checked Fleabay - search motors under O Scale trains, pages 1 & 2 have some possibilities --this is what i'm thinking of http://www.ebay.com/itm/LIONEL...;hash=item232ea0da92 - Found on the second page."
This was made with the understanding that the rear truck would be powered. i have a feeling the wheelbase would be pretty close to the rear one on the McKeen cars; unscrew and remove the coupler assy; grind flat the plastic side frame detail; build a prototypical looking side frame detail out of scrap plastic and brass and glue it to the power truck. Job done!
Madison -- The truck i referenced on Ebay was meant to be used as a complete assy. for the REAR. But you seem to like doing things the hard way in spite of the decades of scratch building experience that myself and others here have offered you in the way of suggestions. In the same search on Ebay on either page one or two were some gearboxes similar to NWSL ones; did you bother to look at those?