It was used on the tender of the ATSF class 5001 and 5011 2-10-4. See page 94 in the Model Railroader Cyclopedia Vol 1. They were oil burners and a tender full of oil and water is heavier then one full of coal and water. More wheels spread the weight for lower axle loading.
I've been looking for them for some time but as far as I know they do not exist as freeware. They do exist as payware but any use of them other on the payware item (and possibly only for your own use) would be a violation of the license.
It is possible to carefully put two 6-wheel buckeye trucks together in such a way as to simulate the 8-wheel one. Looks ok from a distance but a little hokey close up.
