I wasn't going to comment, but I agree with Joe and Jay.
I did my "Log Mallet" with roller bearings on each driver axle. Understand, I am not a world-class machinist, but I make my own mainframes. A ball bearing must be precisely aligned, or it will have more friction than a solid bearing. My alignment was not good enough; the six axle Mallet runs ok, but not as good as similar solid bearing models.
A sprung or equalized truck will give you fits. The bearing has to "rock" as the wheels go over bumps.
A famous collector wanted to commission me to put ball bearings in his PSC Pullmans. I turned him down. He said "you don't understand; I will pay big bucks for this!" I still said no. (It wasn't Jim).