For decades, perception of the big problem the Germans had with Soviet railroads when they invaded in 1941 was seen to have been converting their non-standard gauge (track width) to that of the central European norm. But that perception has been wrong, at least according to one recently declassified report. The document was written for the U.S. Army shortly after the war by Hans Klein, who served as the German army high command's "Technical Officer for Operational Railway Transportation" during 1941-42.
Since the gauge conversion required going from the wider Soviet track to the narrower German, Klein explains, that change posed what was really the simplest of engineering problems for the invaders to solve. Along stretches where the retreating Soviets hadn't had time to thoroughly destroy the rail beds (which was most of the time during the blitzkrieg Barbarossa campaign), all the Germans had to do was pull up the spikes, move the rails toward each other a bit, and spike them down again. No surveying, blueprints, or new construction was needed.
The real trouble was the Soviets built and ran wider (and longer and heavier) locomotives. Those locomotives, being so much bigger than their German counterparts, were therefore able to carry more fuel and water and thus could go much farther between service stops. For example, the average distance between Soviet service installations on the Brest-Litovsk to Moscow line was 138 kilometers. When the Wehrmacht moved in, their railway troops had to build from scratch one major service installation between each pair the Soviets already had in place. Those stations had to include locomotive sheds, repair shops, slag pits, turntables, sidings, water towers, etc., and needed skilled labor and scarce heavy equipment to complete them. (In comparison, track gauge conversion could usually be accomplished employing only primitively equipped conscript labor.) Almost none of this had been planned for, Klein says, and the resultant confusion and delay was the real drag on German railway utilization in the east in 1941.