IMHO thats an out-dated understanding of modern military logistics. Virtually all modernised militaries nowadays are equipped and trained to operate "off road" for most of the deployment specifically because of logistics issues caused by being forced to stick to existing infrastructure, as shown in dramatic fashion by the rapid defeat of the Iraqi Army of '91 which showed the world what a modern Army operating away from roads, at night, in all weathers with rapid logistics tucked up right behind it can do to a 70's trained/operated Army which relied on good weather, daytime and roads to move around on, leading to events like the Highway of Death and the Armoured force left pincer move through the desert into right flank of the Iraqi Army etc. All the ex-WP countries saw how far behind the US they were at that point and any US "enemies" had to change how it operates once they saw that defeat.
So I don't think we start to see Russia suddenly starting to build new roads or rail-yards/rail-lines upto the front lines a few days before an attack. They don't need to, they'll just follow their current doctrine and keep all movement both ways "off-road" once it kicks off for the front line forces, including all supplies which'll be trucked in as usual and any time critical things would possibly be heli-lifted from further back rail heads to supply depots closer to the front (those will be the only things getting bigger right now). It's one of the reasons they've picked this time of year to potentially carry out an attack, as the ground is now starting to firm up after the Ukrainian winter which usually turns the land into a boggy nightmare, making off-road travel much easier on the wheeled vehicles (scouts, supplies etc).
Thats just my opinion though, doesn't mean I'm right.