If fuel and ammo can be supplied 400 some miles from a rail head why can't the iron for railroads be moved forward an additionial 80 miles or so from a rail head?
You realise, of course, for the most part, the Germans had to move NO rails from the railhead?
Russian Gauge = 5'3"
European Gauge = 4'8.5"
All the Germans had to do was move one rail 6.5" closer ... which could be done with hand tools!
Converting the gauge wasn't the problem!
There were a number of limiters ...
* Russian rails were rated at lower loading weights than standard European weights, so they couldn't handle full load trains (partly rail thickness/weight, partly the fact that the underlaying sleepers were further apart). To fix this, all the rails would have had to have been replaced, and at least as many sleepers as were already there would have been needed.
* Because the Russians used a wider gauge, their locomotives were bigger and had, therefore, bigger water tanks ... so they could go about twice the distance, or more, between rewatering and refuelling than European/German locomotives. Even assuming all the water towers and fuel facilities survived, the Germans then had to build another one in between each existing one.
* Also, another function of the bigger Russian locomotives was that they needed maintenance about half as often as standard gauge models ... so maintenance facilities were about twice the distance apart as in Europe. Again, even if captured intact, it meant that the Germans had to build one in between each existing one (and, unlike the water towers etc. this is all special order equipment, and you have to wring its priority away from Fat Herman, who controlled allocation of all iron and steel!).
The original plan was to capture lots of Russian locomotives and rolling stock intact and use *it* ... but the Germans found that soldiers really, really, really, really loved nothing better than shooting up locomotives and watching their boilers spew steam, or even cause steam explosions ... hence, though they *did* use captured rolling stock on some sections of track, there was never anywhere near enough (and there wasn't enough standard gauge stuff, either, and they didn't have enough capacity to divert from tanks and guns and planes, which Fat Herman prioritised anyway, to less glamourous stuff such as locomotives and rolling stock!)
Author, Space Opera (FGU); RBB #1 (FASA); Road to Armageddon; Farm, Forge and Steam; Orbis Mundi; Displaced (PGD)