It would to some extent because it would allow the Germans to use ships (ships being an even more efficient way of transporting cargo than railways) to shift bulk cargo as far as Leningrad. This would allow extensive operations around that area and an assault up the Kola Peninsula. How far that offensive would get is an interesting question; my private bet is that it would run out of steam around teh White Sea Canal. Kola is superb defensive terrain. It would also allow German operations to proceed towards the White Sea and may add Archangel'sk to the long list of heroic Russian sieges (this is what happens in the book )Thanas wrote: Would taking Leningrad alleviate the problem? Because it seems to me that with the baltic ports in German hands, they would have an easier time supplying their forces, at least in the northern sector. Seems to me that this is the only way the Germans could somewhat influence their supply concerns.
However, it would not affect the operations further south; the conformation of the railway net simply doesn't allow for it. There are three railway "lines", one supporting each invading army group and each is pretty much isolated from the rest. By the end of 1941, each was maxed out in terms of capacity. So, eliminating the use of the northern line for primary supply doesn't make mroe capacity available on the central and bakerloo southern lines. It makes more rolling stoke available but the line capacity problem means that rolling stock is less than usable. Having said that, eliminating the northern line would free up engineers to work on the other two so they would push east a bit faster.
So, overall, I would say a serious improvement in the north and a marginal improvement inc entral and south. However, that assumes the docks in Leningrad are usable. I rather suspect they might not be. Not for a long time anyway.