".... It is not a question so much of the Germans getting working oil fields as it is the Russians loosing them. What other large sources did the Russians have at the time or would they be almost entirely reliant on LL oil products? "
None, at the time. Once the Caspian fields are lost the USSR will be dependent on the USA for all their gas, diesel and lubricants - but with other priorities shifted, the USA has the capacity to supply, IMHO.
"... How much of the Trans-Siberian RR would have have to have been upgraded (better roadbed, heavier rails, more parallel tracks in places and or more/longer passing sidings, newer bridges) to handle a much increased flow of traffic? yes it could have been done and most likely would have been done but then without the Moscow rail hub distribution of material and men to the northern and southern parts of the front would have been much more difficult or the rapid transfer of forces of to reinforce defenders from attacks or to reinforce a successful local attack. These problems could be over come but, again, it is going to take a considerable amount of time, effort and aid."
In 1945 when the August Storm build up began - the Soviets left most everything in place in the West except Medical, Engineering and Communications units - they ran 100 trains a 24 hour cycle east (according to David Glantz), IIRC, in addition to trains running from the Pacific westwards carrying LL materials shipped from the USA.
The Russians build wider scale railroads for a reason ....
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MM