I was thinking the various countries used a different mix of metals to make their track. How about a track color set. The Germans had a darker metal. I am not sure about the Soviets, Italians, British, Japanese, French or the United States. There are blackening agents for metal but I truly suspect that there is a lot of artistic licensing going on. Yes, you would still need to weather/rust etc but you would have the correct starting color and not just black, steel or rust.