The original Weaver RS3s had a sprocket tower in the tank with shafts out to both trucks. The motor was in the hood. It would go around 24 inch radius although the pilot over hang was so much it could not pull anything. It could work with 40 ft cars on 36 inch radius.
While the motor was on in the tank, the drive train was.
And by the way E7s are not particularly small locos.