The RFA is fighting in battalion tactical groups, and these BTGs have a minimal capability to keep themselves supplied. For this purpose, each of tactical armies (every tactical army is controlling between 2–3 and 20 BTGs) has its own supply brigade. The number of such highly-specialised units is very limited: I think there are only 10–12 in all of the RFA, and they rely on trucks. However, this is not as important as it might appear because the RFA is even more dependent on the use of railways to keep its troops supplied. Not sure about the total number of railway brigades of the RFA, but think to recall reading there are about 20 of these.
In this war so far, the shortage of supply brigades resulted in a situation where every single tactical army was capable of simultaneously supporting the advance by only one BTG. But, in the Popasna area, the RFA is running an offensive involving some 5–6 BTGs.
How comes?
It’s simple. Contrary to the situation elsewhere in Ukraine, there is a excellent railway network in the Kadiivka-Alchevsk area, only 30km east of Popasna — i.e. in the area controlled by the Separatists already since 2014.
Moreover, by taking Popasna and then Volodymyrivka, the Russians have secured an additional branch of railway, which they can now connect to the system in the Kadiivka-Alchevsk area.
Pay attention: the RFA advance from Popasna on Vrubivka is run along another branch of railway, too.
With other words: the way this operation is run, they can bring their supplies by train straight to the frontlines. No need to try doing that by truck convoys: not only is the RFA meanwhile critically short on trucks, but its truck convoys remain one of primary targets for the Ukrainian artillery.
‘Problem solved’, and then a crucial problem — that of logistics. This is why the RFA is proving as capable of successful offensive operations in the Popasna area: its artillery can continue blasting Ukrainian positions day and night, the Guards’ Terminators can’t run out of ammo for their 30mm guns…