I decided to investigate the supply push from structures to distant forces with supply trains, as this aspect is poorly documented in the manual.
I played the first 6 months of the grand campaign both sides by just placing a couple of forces with trains at various distances from structures.
In general it behaved like I have read here on the forum, forces at three regions (two intermediate regions) distance with a road will remain supplied at 80% to 90%, three regions through rough terrain is blocked (although I noticed in an earlier game that three regions through woods towards Kassel would pass supply towards besieging forces.
What surprised me was that a besieging Prussian force remained supplied at Köningsgrätz while the mountain passes to Silesia were blocked by winter for many months. They evidently didn't block the supply line.