I am not trying or intending to twist your words. Of course ESE (supplies) can flow along any route. The point I am making does not relate to Berlin-to-Rostock. I am discussing human player knowing where best to put his efforts to better ESE for his Wehrmacht deep in Russia. If the supplies are coming from Berlin why are you mentioning Rostock? Obviously supplies can flow across every map province, and supplies do flow all over the place if the army is close - like start of Barbarossa where the supplies will flow over every province in Poland because of the spread in destination points.
However, when you get a situation of your army deep enough, like waiting in southern Mongolia, all the supply will flow down one line of provinces only to the entry point of Mongolia, and then spread out to the final different destinations. But if TC overload is reached some of the extra needed supply flows - like water - in a river that has over flown its banks. There is not much the player can do about that because he cannot fix all of Russia.
The point of building railroads is to get the maximum supply down that identifiable route - which can be improved. Of course, when supply gets close to Mongolia it will branch off at a certain point and each branch finds its best final route to the different units throughout Mongolia - be it northern or southern Mongolia. This does not affect that all that supply - if it is not overloaded demand - flows from Berlin precisely thru ROSTOV in Russia (not sure why you mentioned Rostock next to Berlin). At this point the supply flow will branch to find the shortest route whether going to Baku, or a southern route across Russia to Mongolia, or a quite northern route if going over the top of Manchuria. As you approach the Far East coast it will branch again - south to Vladivostok or north depending where your units on the coast sit. And further, any supply to any unit off the railroad anywhere along the line will also have its supply trace leave the railroad at some point. But the point is - very a very long ways going from Berlin - it travels along 1 identifiable route - which can be improved. If ypu could not find the route, how would you know what/where to improve it. Back to indicriminantly scattering garrions around?
An easier example is supply to Burma because the different countries are more relatable than discussing the many options in Russia. The important fact is that all this variance in supply flow depending on where your troops are stationed has nothing to do with the fact that if you want good supply reaching Burma there is precisely ONE line thru the Caucasus and over Persia - until you get overload. Improve the very low infra provinces along that line where they exist in the Caucasus and thru Persia and you will delay overload occurring. Naturally, at India supply flows differently to all the various coastal guard units. But as regards supply going to Burma there is only ONE precise supply trace across India (a quite northern route) for the supply going to Burma. Knowing that, you now know where in India precisely to place GAR or CAV to eliminate partisans in that supply trace line. If India had any low infra, you would also know where to improve it. As it doesn’t, player should concentrate on improving the low infra provinces on supply trace to India back in the Caucasus and Persia. OF COURSE, if you overload TC, supply will flow alongside the intended supply line. So improve the main supply line – once it is found – and it will be what is used for all supplies as long as overload is not reached. The point is you can greatly mitigate against overload by improving the railroad at its correct places as regards Berlin supplies going to Burma.
And as regards going across Russia, there is a main northern route and a southern route depending how deep your units are in Russia. So there is very much a player can do by knowing where those lines exist when supply is flowing far across Russia. And, unfortunately, the supply trace to deep Russia - for any destination in Mongolia or Siberia or the Far East - goes right thru 20% infra Elton – ONE PROVINCE – and not around ELTON via three 60% infra provinces. Of course if you overlord then it will spill over across those 3 additional provinces. Would you rather improve ELTON so its infra can handle the demand and eliminate partisans with 1 GAR in ELTON, or would you like to place an extra 3 GAR because the poor infra in Elton has caused the supply to spill over out of the direct route and is now spilling across an additional 3 provinces skirting ELTON? Now you need 4 GAR total just to deal with getting supply past ELTON because the first supplies (if no overload) will always flow only thru ELTON (if headed for Mongolia, Vladivostok, or Kamchatka).
A screen shot would really help, but I won't do that for this thread. But it is kind of nice getting this discussion out of the way. The only relevant point as regards what player can do to improve things is to learn the precise line for a very long supply destination – and fix the poor infra and revolt risk along that line. That is the concept of a railway – to give the supplies a reasonable easy path to flow so TC is not overloaded because – when TC is overloaded – then supply flow runs helter skelter extra to the main line. Improve the line and you enhance avoiding overload.
The only real question is how to determine where is the line which is the only line that will be used as long as TC is not overloaded – so you can improve it, and run greater amounta amounts of supply along that route. I am discussing supply trace from Berlin to past the Ukraine at least and not discussing the variances in supply trace that occur as supply nears destination and then runs over all final provinces to reach the different destinations as per an army spread out along a front. To clarify - this is "the railroad to Russia and thru Russia", and not the helter skelter flow of supplies as it branches near the ends to different final destinations. And that railroad includes ELTON - the most serious obstacle in the long railroad line. SO, "YES" improving ELTON infra is greatly helping me getting more supply to my forces in Mongolia and also all of the Far East because - at Elton - there is still only one line for those far off destinations (unless you spill over because of overload).
It really is not that complicated. But to map it out in color for screen shots that are very self-evident you need access to Russia - meaning you conquered it. However, anybody can do it anywhere. You can trace the supply route from Berlin to Paris if you want, and mark it in color using the blue of province priortized in Supply Map Mode. And as long as there is no overload, supplies from Berlin to Paris will only flow in exactly ONE identifiable route. Now, if there was a low infra province between Berlin and Paris, you have the needed visual map reference to fix it. But as there isn't the discussion goes back to Russia with its very poor infra in many places and locating that very long railraod from Berlin. If player could not do this there woud be no chance at all to know which of the couple hundred provinces needs fixing. What is the point of any discussion of "railroads" if you can not trace out the routes on a map? Simply have a unit in Berlin and mission him to any point. And that is the route the supply will also flow - unless you are overloaded in which case the overload demand starts spilling thru adjacent provinces. It is not rocket science but the only logical choice as to how supply can trace across a continent.
But when you do this with an all conquerd Russia then you get some very interesting info and views.