Those Kadee washers are hard pressed fiber, and will work fine as gear shims. In fact, Lionel used to make some of their O gauge motor gears from pressed fiber back in the 20's and 30's, and they held up very well as long as they were made properly.
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I wonder if you could explain (preferably with photos) gear shimming? If you're talking about taking end-play out of worms, NorthWest Short Line makes thrust washers in various sizes and thickness. They're bronze and made specifically for such work. They're also useful for removing side-play from other types of gears. If you need to shim a gearbox (increase the width of a split-sided one) or increase clearances for a bottom coverplate, brass shim stock is available in increments of .001". You can cut this material with a knife or scissors, at least in the thinner sizes.
Wayne