I have a previous question that led to this one. There is an article talking about German rail activities in Russia during WW2, and it also talks about the state of the Russian rail network to begin with.
Near the end of that article, it says:
Whereas German and most western rail bed construction methods contained a multi-tiered rock and gravel foundations - Soviet rails were almost always sitting only on a bed of sand covered occasionally with rocks to minimize the inevitable dust clouds. The western regions of the Soviet Union suffered a great rock shortage. Deutsche Reichsbahn - The German State Railway
I did not see a source for this or anything else in the article. I bolded the last part because I'm interested in it.
"Rock shortage" means a shortage of crushed rock ballast that's put underneath railroad tracks. These crushed rocks serve as a foundation.
I am not an expert on geology, but I have the read a little about these kinds of things and am led to believe that Limestone and Dolomite are basically "everywhere". I do not see how West Russia could have a shortage of them.
This could lead me to guess that the rock shortage, if it really existed, was artificial in nature. By that I mean that the industries in Russia were focusing on other things besides improving the rail network. But this would not be my first guess because Russia is an "Empire of Land" for lack of a better term, so you might tend to think the railways were prioritized.
I also note that the article explicitly said West Russia without further explanation. So I could guess that Siberia had all the rock, or I could guess that it's only West Russia because Germany never made it beyond West Russia... The article also does not put a date on the rock shortage, so it's hard to guess if it was 1920's, 30's, 40's, or maybe all that time.
So you can see I have many questions about this article, primarily because it doesn't have enough detail. What was the exact nature of this rock shortage, if it really existed? I suppose you could say that I'm also asking for an alternate/corroborating source.