Hammond Lumber in Belgrade is probably the only potential customer on that stretch
Yep. The one and only. And at only a few cars per delivery, that's not a customer worth keeping in the event of a mainline swap. The Madison Branch has been out-of-service since 2012 with no potential customers ever since the big paper mill closed. PAR has already been raiding that ROW to rip out newer lengths of replacement rail for patches elsewhere on the system, so the only branch between Leeds and Waterville is also 99.99% dead pending the new owners belatedly filing the abandonment paperwork.
Prior to about 1985, Maine Central forked the mainlines at Yarmouth and sent loaded cars Portland-Waterville up the Lower Road because of its easier riverfront grades, and ran mostly empties on the hillier/more-desolate Back Road where they could take coasting advantage of some of the down-grades. So there wasn't really any on-line business there historically...just the branchline jobs to the paper mills of which "PORU/RUPO" (Portland-Rumford/Rumford-Portland daily) is the only survivor. The only reason PAR chose to get off the superior route in favor of the inferior one was to get out of spending money on some Lower Road cycled maintenance that was due. No cost too chintzy to shoot one's self in the foot ducking, in Tim Mellon economics. Right now the Back Road has decayed to 10 MPH and takes an entire day to get a freight train between Portland and Waterville.
CSX would definitely be all-in on a shotgun deal with
Downeaster expansion north of Brunswick to regain the superior mainline routing and shed some costs in the process. Dumping $$$ down the black hole to pull the Back Road up to even 25 MPH at *borderline-acceptable* state-of-repair is a fool's errand for the meager ROI (albeit one they may have no choice but to do if there's no other way). Partnering-up on new mainline infrastructure may be the sum total difference in CSX/etc. choosing to reinvest in Waterville Yard as a major I-95 intermodal trucking site vs. simply slashing it down to the barest possible ops-minimum as a crew-change point on the slow 2-day sojurn between Portland and Bangor. And that has large implications for Northern Maine's economy, so the alphabet-soup agencies need to read the needs behind that
DE Augusta/etc. extension in three dimensions for how the freight economy stirs the whole drink. As is, Waterville is going to get broadsided next year with mass layoffs as PAR's World Headquarters engine shop gets shut down post-merger with all of those high-paying shop jobs relocating to Albany where CSX services its Northeastern fleet. So either they find some strategic way to exploit ROI up there or the rest of the downtown yard could be next on the chopping block gutting out Waterville's downtown from the riverfront on out.
Even if passenger-funded upgrades stopped at a Phase I Augusta, that's still enough Class 4 track to slash half the travel time off a Portland-Waterville freight right then and there. New owners can then piecemeal-upgrade the Augusta-Waterville half (called the "Augusta Branch" today for the 3x a week local that runs as far south as the feed mill in Augusta) to 25 MPH. Thanks to PAR destroying most of Northern Maine's customer base over 40 years it wouldn't have to be double-tracked most of the way to easily accommodate a
Downeaster schedule with 40 MPH freights; most of the trans- Portland/Waterville business is wadded up into a single daily round-trip that's long in car length but self-contained within its slot...so congestion is several orders of magnitude lower out here than it is on the Western Route south of Portland. Running nonstop on good track, the Portland-Waterville transfers also would stay well ahead of any
DE's making local stops, so passing tracks would only be needed at opposite-direction meets (and the Lower Road, despite being majority single-track, has luxury of many former passing sidings) and around station platforms for installing full-highs (a la Brunswick).
The Back Road to Leeds won't be going anywhere; the two paper mills in Jay and Rumford are still healthy, and the intermodal yard in Auburn at the interchange with St. Lawrence & Atlantic (double-stack, 286,000 lb. lane from Montreal) is one of the prime 'gets' in this deal for CSX since they'd invest in more traffic at that site in a way half-assed PAR never dreamed of. So the portion of that line that matters for future Portland-Auburn/Lewiston or Portland-Montreal passenger traffic will be busier than ever and kept to better state-of-repair than ever even if that most-desolate stretch of track between Leeds and Waterville gets axed for a rail trail with the mainline swap. Traffic north of Portland has always forked in a "V" shape northwest vs. northeast....PAR just made a false choice sacrificing one side of the "V" for the other all those years ago, and making
all endpoints on it inaccessible during a day's work from their negligent maintenance practices.