How much railway supply did they need?
They were never more than 50 miles from their own border...
They were using the Belorussian railways to feed their Belorussian railheads at Gomel and Mazyr. Thats where (in Belorussia) they transferred supplies to trucks.
They were a lot further than 50 miles as they had to drive a couple of hundred miles into Belorussia and across the Dneipr to support the West Bank operation to Kiev. Look at a map.