In my experience, the thicker the plywood the less the sound amplification.
The wheels make a huge difference too. I have two otherwise identical Blue Box boxcars, one of which was equipped with metal wheels, the other which still has the plastic (probably the first BB kit I ever assembled - like many people I never painted the weight, so this car needs a little work before I put it on the layout). Rolling the cars back and forth, the plastic wheel one is almost silent, the metal wheels have that ssssssssssss sound as it rolls.
Denser materials do not vibrate as much at the annoying frequencies - I had a layout preciously that I built on shelf brackets, it used the high density particle board - the stuff that doesn’t really look like it’s made of particles, it’s VERY heavy, it’s hard to cut (I had the pieces pre-cut), but the layout was pretty quiet with nothing more than cork on top of this board. Likewise my test track, it’s just some track sitting on cork on top of one of those Rubbermade laminated shelf pieces - quite quiet. Not that my 2 layer foam plus plywood is loud, certainly not in any objectional way.
–Randy