I cannot offer an expert opinion on the driving wheels. I can only say that I have seen everything from this style to solid to cored spokes and seemingly every variation imaginable. It may have to do with railroad preference which was not always rooted in sound engineering practice, or it may have to do with the foundry practice and preference of the time. The latter seems more likely. You'll notice that the rims are divided into sections which was common around the turn of the century. It seems likely that this was because of the differential cooling rates of the larger hub and counterweight sections compared to the thinner spokes and rim though I had one good present day foundryman tell me it would make matters worse. Our new wheels are cast with chills in place in the larger sections which allows a more even cooling and shrinkage throughout the casting. I have not seen signs of these in old wheels I've looked at.