The more I see, the more it looks like sabotage.
Russian forces now have most of their rail supply lines cut in Ukraine (in the north, the east and Crimea) and that is how they move much of their military equipment and ammunition around. If Ukraine can cut the last railway connections in the Donbas, then the Russian military supply chain is completely broken.
The Russian army faces a horrendous winter with the possible surrender of its entire force.
I think the Ukrainians are looking to deliver a quick win now. Exhaust the Russian army in Ukraine of its ammunition asap (weeks?). The pincer is all-embracing. Probably the least bad option for the Russians is to retreat by road into Russian territory. But General Putin won't allow it, of course. And the roads will be choked and a helluva target.