Quoting Nathaniel Brill
Do you notice a significant difference with only one flanged driver as opposed to two?
The single flanged driver doesn't adversely impact anything that I can tell.
This model would not run around corners or switches with two pairs of flanged drivers as the second pair of wheels on the lead truck are fixed in place. On the "lead truck" only the very first axle pivots relative to the body of the train. This allows the second axle to be tucked in close to the drivers and enables a very compact design.
As far as I can tell, Sava is the one that really came up with that technique. I didn't fully appreciate how elegant that solution was until building this engine.