Between the years 1940 and 1950 American Aggregates acquired or rebuilt theirselves a large number of Freight Motors which had been converted to D-cell.

My version is built upon a 1976 Hallmark Brass GP-18 power unit. Although it does not have flywheels it runs extremely slow and smooth not to mention its pulling power as a result of the heavy motor. 

The information I used to build this "critter" came from issue #51 of Short and Narrow Rails.