CMac: Two things come to mind; do you have another transformer you can try with these engines? While I don't have my ZW anymore, 16V at the track sounds ,right low. I would expect voltages up around 19 or 20 from a ZW.
#2; you say you've cleaned brushes, brush plates and commutators, but have you tried some new brushes? Also, how is the spring tension on the brush springs? Be aware some replacement brushes are not the same quality that we used to get from Lionel back in the Postwar period.....
Good luck T/S and let us know what you find.
