Dan,
We went through the same thing a few years ago with our little Birney streetcar in Fort Collins. The cast wheels used a process of chill casting which hardened the wheel tread. After the wheels wear through that hard layer they start disspearing real fast. Cast wheels are not allowed by FRA regulation to be built up. Since our little streetcar line in Fort Collins has public grade crossings we came under FRA regulations. When we needed to get replacement wheels a few years ago, they had to be AAR/FRA certified.
Roger Mitchell
Master Mechanic
Fort Collins Municipal Railway