As far as the train supply issue goes: A basic rule of logisitics is that, assuming the same supply point, if you double the distance which you are supplying you more than double the amount of ships/trains/trucks needed to keep a unit going. This is because a train running, say 500km, can make 1 full cycle per day. If you double the distance, now the train is only making 1/2 of a full delivery per day, plus is consuming 2 days of fuel per delivery. Or, effectively, the operational cost of that train per load of supplies has quadrupled. You spend 2x the maintenance, and coal, but only get half the stuff. If you quadruple the distance (the situation on the Russian front) your trains are 16 times less efficient than if you are operating outside of Warsaw.
That means that for the germans, supplying a division outside of Moscow costs them ~5-6 times the logistics capacity as supplying that same division outside of Warsaw (or in Belgium). That is just brutal and the single biggest reason you can't knock out Russia in a single major effort. Russia could *only* manage supplying their own troops because of the unique railhead situation; with Moscow the effective distribution point (where much of the military supplies were piled up) the effective supply range was manageable. Feeding moscow with supplies was a challenge, but one that the russian rail network was already designed to handle.
As the germans advanced, the Russian supply situation got better and better, while the German got worse and worse.
Hitler would have been better off building an effing buttload of engines and rolling stock, rather than Bismarck & Tirpitz...
*edit
I should note, that is why I consider taking Moscow the single critical point of the entire campaign. It makes almost as big a mess out of the Russian supply situation as the German one, and, potentially, allows the Germans to establish a single 'dump' to support operations both in the north, center, and south. That means you ship your daily 'expected' needs directly to the N/S fronts, and everything else direct to moscow. From moscow, you get a whole new line of logisitics to build up for a push.
That is, if you hold Moscow, you can supply your troops outside leningrad along two major rail lines, rather than just one. That actually lets you build up enough material to launch a major offensive.