Plus Acela/Acela II would require electrification all the way up to Portland.
And that's likely not possible on the PAR freight main, which will be cleared for double-stack loads by this point and will need 22'6" clearances to run those freights under 25 kV wires. It's going to be hard enough just getting between Andover and Haverhill with wires for RER the way some of the overpasses between Lawrence and North Andover will be utterly maxed-out of further space. It's not a much prettier picture in New Hampshire, either. Besides, it's not an intrinisically very fast route with 80 MPH or less being the geometric max sustainable...far too clunky to attract flies for a premium service. The
Eastern Route was always the faster of the two BOS-POR schedules back in the B&M era. It's just that since the Western Route carried all the profitable freight it got better maintenance than the Eastern, and then when finances got tight the Eastern was the easier one to drop.
If *fast* service to Portland is desired then re-routing via the Eastern out of Portsmouth is the way Amtrak service will need to be restructured. Wires are feasible the whole way, no freight interference, most of the ROW Revere-Beverly, Beverly-Ipswich, Ipswich-Newburyport, and Salisbury-Portsmouth is arrow-straight, and the grade crossings are few and far between and mostly limited-concern. Not to mention a stop roster that swapped out Woburn + Haverhill + Exeter + Durham for Salem + Newburyport + Portsmouth would be a net-gain on population densities served.
To do it you'd first have to have a commuter-only plan in place to extend Newburyport service back to Portsmouth. Not an overly involved undertaking since the trail segments are mostly out in wide-open marsh areas where relocation to the side is negligble-concern...while the shortie downtown Newburyport trail can be relocated a couple blocks east next to a traffic-calmed Route 1. You'd have to rebuild the derelict swing bridge, though an engineering survey done about 15 years ago found the approach spans to be in good condition for refurb-n'-reuse framing a replacement bascule or lift span. It's mainly that NH is such a hot mess for transpo planning that no one realistically expects them to be able to tackle this one before the heat death of the universe (they're having enough trouble even getting a train across the border into Nashua). The T from its own standpoint would probably have this one stuck to a long-range wishlist by now were it not for the NH build dependency making that a futile gesture.
Get to Portsmouth and rather than crossing the rail/road bridge into Kittery on the old Eastern alignment, continue west on the active Newington Branch to where it ends at the Piscataqua River. The remainder of the branch to Dover was torn up 5 decades ago to build Route 16, which runs on the old railbed from the river to Exit 7 (NH 108). You'd have to build a new river crossing, then construct about 5 miles of new ROW on the highway shoulder (ample room on either side). Widened overpasses, etc...but since NHDOT loves to bankrupt itself on add-a-lane projects that's right up their alley. At NH 108, transition back onto about 1 mile of legacy ROW to the old junction at Dover station. Or, if downtown Dover has some NIMBY's...continue along the highway for another mile and meet up with the Western Route a little further downstream from Dover Station.
Dover-north you probably *can* tackle electrification of the Western Route to Portland owing to the relative scarcity of overhead structures. Some tricky ones, but for the most part just overpasses where the trackbed can be undercut to gain the extra headroom. There are 2 more excellently well-preserved abandoned segments of the Eastern Route (North Berwick-Biddeford, 16.5 miles + Saco-Scarborough, 7.5 miles) that pull off then back onto the Western. They're each arrow-straight, crossing-few, more-or-less a wash on same ridership catchments served (with exception of Saco-Scarborough bypassing Old Orchard Beach), and capable of sustaining >100 MPH speeds. But I doubt the Maine economy would sustain restoration jobs of that scope where Amtrak would literally be the only user. Those are going to have to be much deeper future considerations, with nearer-term focus being just ramping up the speeds through MA/NH with the route relocation and then doing what you can do in ME to make a wired-up Western good enough on its existing alignment.