There are a few other photos of this train.
The spokes appear to be a tapered, rounded, rectangular shape, wider from the center of the track to the outside of the track.
There would be some fillets as well.
Outside view would help establish their other dimensions seen from the side, and perhaps some of the fillets.
The inner and outer hubs, and the (10?) spokes, were likely cast in one piece.
I think that the outer rim, which contacts the track, would be designed to wear down, and be replaceable. (?)

Could not locate any plans on the web...

- Brian

William Buchanan was the designer of the "999."
40 inch tender wheels?
https://www.msichicago.org/explore/whats-here/exhibits/transportation-gallery/the-exhibit/999-steam-locomotive/