Yes, rail beds were a problem for the Germans...Only the main lines used a stone/gravel/rock bed, while the other lines used a sand bed or no bed at all - just tracks & rails laid on the ground. Bridges were another problem, in that they were not well constructed and could not take the heavy loads the Germans intended to haul on them. Not that it mattered, near about all railway bridges were blown up as the Soviets retreated. A good overview of railways in the East can be found at Feldgrau: Deutsche Reichsbahn - The German State Railway And one of the best AHF threads on the matter: https://forum.axishistory.com/viewtopic.php?f=66&t=203286
Get a copy of "The Most Valuable Asset of the Reich: A History of the German National Railway Volume 2, 1933-1945" and look its source material citations. That and search the bundesarchiv.