You need a very strong business case to get the branch going again, considering it was abandoned before the mill closed which in itself would likely have been a stronger reason to rebuild the branch.
The 180,000 tons per year of softwood chips will source from local wood waste of saw mills, biomass, and other sites. But to remember, assuming nothing much is lost in the process, inputs = outputs, so we know we'll get at least 180K tons per year of finished product, which I suspect will be slightly more with other raw materials mixed in. So in this case, you're looking at 9000 truckloads per year, something around ~25 at full output. Could rail outbound rail supplement truck capacity? Quite possibly for someone with the right size check book.