>>What made the geared difference was that every wheel on all the pivoting trucks was a driver

I think there might be a bit more to it. Rod-driven locos, have, essentially, one connecting rod from piston (linear motion) to the wheels (circular motion). From what I know of internal combustion engines, that means there will be parts of the circle in which the rod has very little mechanical advantage, so, effectively "dead spots" even if the piston is driven in both directions and on each stroke. In an ICE, the solution is a flywheel and/or more cylinders with connecting rods out of phase so there's always one con rod producing power. The Shay engine does similar.

Just thinking it through...