This may have been the doctrine during the Cold War it is very much modified now. Trains are the prime movers only really in the base country to get equipment to ports which then deploy to the closest seaport or landing area to deploy the equipment cheaply and with minimal disruption to civilian traffic. While prepositioning equipment is still used it is not as prevalent because of vulnerability to not only attack but being not in the right place and usually having to bring the people needed to operate it, which means the equipment is not instantly available as it typically needs to be inspected fueled armed ect. The US military has found it is better in many regards to simply ship the units equipment and operators together. Trains are not really used in theaters anymore equipment is moved by truck mainly in theatre for two reasons flexibility and redundancy. The trucks don't have to get modern equipment all the way to the front just close. When one breaks down the rest still move and if a road is blocked or destroyed trucks have an ability to make their way around similar to how ants work. Another reason that is not mentioned much is parallelism of movement whereas trucks are not restricted to just main roads but can utilize smaller arteries to spread the logistics train in such fashion as to make it difficult target and cut significantly in volume.
Your post that's swarmed into a single hive makes a number of valid points such as rail infrastructure being used in the base country to get heavy weapons and other vehicles to port and shipped out. Yet for NATO countering a Russian cross border advance into the Baltics NATO would be receiving not shipping.
And if Russia can obstruct, disable or preclude shipping to Rotterdam and other key ports along the Baltic Nato is going to be hard pressed to get its hands on the heavy weapons and equipment it will need to confront the Russian invader successfully. Perhaps some Baltic states ports could be held open as in Pusan in the Korean Conflict but their capacity to receive is definitely limited.
Which brings us back to air superiority which you don't mention in any way and that is the first priority of Nato air forces, accomplished namely and as I pointed out by the US, UK, France, Italy. Allied air superiority as the first strategic order of priority would enable their air attack forces to penetrate the Russian base homeland to destroy their rail infrastructure network. Indeed Russia needs it rail system and network more than Nato needs its own.
Nato's first challenge to save the Baltic states is to fight its way through Kaliningrad while holding off or repelling Russian troops traversing Belarus, which only a safely ashore second wave can do. It's the second wave that would use prepositioned stores of vehicles, ammunition, supplies -- beans, bullets and bandages -- as it's a given the forces that are the first responders haven't the time, resources or personnel to crank up and move the stores in the opening hours or days.
So rail transport and delivery via the Russian base is much more of a vital factor to the Russians than it is to the Nato allies. The challenge to the Allies is to get its stuff and forces ashore via viable ports and to sustain landings in more than one cycle of delivery, to include supplies and sustainment of course.
Additionally and I don't know but it's conceivable Nato has a Plan X as in cross your fingers to enter Russia via Scandanavia to access the Baltic states by land from their east while staging a diversion or an actual Baltic seaborn approach from the north. While that would be a militarily smart operation it would be extreme, but then again there are many variables to another conflict in Europe-Eurasia some of which could be extreme indeed. One variable would be to give in to Russia which would be catastrophic for sure.
en.wikipedia.org
