I am reading a great book Stahel; Operation Barbarossa and he mentions something about the German plans for the Soviet rail line. The first is the Germans just assumed they would capture the lines and trains intact. When that didn't happen the problem was not just converting the lines to the narrow gage, but Soviet engines were larger and had a longer range and so the Germans had also to build new stations for repair, coal and water. This need to build new stations is one of the reasons for the difficulty in transporting supplies. The second is Hitler had neglected the rail system and there was a shortage of engines and the track system needed major over hauling.