One reason it was so hard for Germany to invade Russia/the Soviet Union in World War II (and I) was because the Russians had a different (wider) railroad gauge than that of most of the rest of Europe, making European rolling stock useless on Russian railways.
How did Germany resolve that problem in World War II? Did it capture enough Soviet rolling stock to use on Soviet railways? Did it build "trunk" lines in the Soviet Union to e.g. Stalingrad and other critical points on the European gauge? Or did it leave this problem mostly unresolved, thereby contributing to the German defeat?