IIRC as delivered the RDCs had various sound- and vibration-absorbing pads at key points in the trucks; the brand may have been 'Fabreeka' which seems to still exist. The character of the Boise Budds' ride led me to guess that their pads had been replaced with solid steel blocks.
I've read about the suspension mods elsewhere, that sounds about right!
Got a bit confused about my thread moving, although I am planning to model the Boise Budds, I'm really only interested in prototype information at the moment...