The reason U33 ad SD/ES wheels are incompatible is the U33C's have a horizontal motor in the fuel tank and the SD70/ES44's have vertical motors above the trucks. Lionel makes no mention in the catalog that the "U36C's" have dual motors. "Powerful maintenance free motor"(singular) is all they list.
Frankly, I think any scale conversion wheels for the U33/36C's (and SD70/ES44's) are existing stock. The only difference I know of is the gearing was different between the original and following U33C runs (which also includes the worm gear.)
Unless something drastically changes, once Lionel exhausts their supply of scale wheels, I strongly suspect it will not be replenished and the "scale option" will fade into history. That's probably why the part number's not cataloged (After all, how much would it cost to take up a small patch of the catalog page for the conversion wheels? It's not like there's no real estate available on the pages.) and you've got to call Customer Service for the secret part number...
Just as scale wheels are supposed to be available for the new run of cylindrical hoppers, but the only p/n I located is for the entire truck, which likely has the original style high bolster and out of gauge wheelsets.
BTW: I would love to be proved wrong.
Rusty
