Acelas already have a lot of space in front of them
Not enough to even attempt the kind of speeds that you get only on dedicated high-speed railroads. The Acela is not that kind of train.
There are two problems with your argument about MN. First of all, they will have ACSES for the 2015 PTC mandate, so that's taken care of, and secondly, I'm not saying they should magically do 180 on the New Haven Line. That's ridiculous. However, they should be able to get into the triple digits on some straight sections, not 75, and rebuild bridges to remove speed restrictions, some of which take Acela down to a crawl. If Acela could blast through the bridges at 70 or 80, that would save a TON of time over the current very slow speeds. This is currently the slowest portion of Acela's whole run, and modest speed improvements to eke out as much as possible from the current alignment are needed in order to keep Acela moving through this area. It would also sort of benefit MN, as the M-8's can run 100mph, and it would definitely benefit the NE Regional trains
No, that's all a pipe dream. The 2015 PTC mandate does not mean that Metro-North will necessarily use ACSES, and it certainly does not mean that Metro-North or Connecticut DOT will ever think of building Class 8 track on their property (don't even think of Class 9 track), or adjust their track centers, to accommodate Amtrak. No flaws in my argument because of those matters. Besides, how are you going to relocate Jenkins Curve?
And frankly, it's
not the slowest or curviest of the NEC. Have a look at the Shore Line east of Old Saybrook and through New London, in Amtrak-owned territory.
You are totally distorting what I said about passenger diesels. Most sections wouldn't be good for 125, but the equipment should be good for 110 or 125, where it is possible to run at those speeds. In most areas, there are a lot of stations, but at least they could open the electrics wide until they have to slam on the brakes for the next station. Also, the electrics accelerate a lot faster than diesels, so they keep a schedule that is less different than Acela's and NE Regional's (although still quite different)
That's another unnecessary expense. NJ Transit's cars are certified for 100 mph, SEPTA's for 80 mph, Metro-North's for 90 mph; it's a waste of public funds to certify them for faster speeds or operate them that fast, even for rush-hour expresses. And "opening the electrics wide" only to "slam on the brakes for the next station"? Great way to increase wear and tear, as well as increase passenger discomfort.
Going to explain what to do with the Empire Corridor (they run dual-mode diesels right into New York Penn Station) and Metro-North's New Haven Line branches (the Danbury and Waterbury branches also use dual-mode diesels during rush-hour service into Grand Central Terminal, where they need to operate on third rail)?
Electric motors don't accelerate too much faster than diesel-hauled trains; the starting tractive effort is very similar. The difference is high-end horsepower, and commuter trains aren't the most concerned with top speed.
The only MBTA line that runs on the NEC and in diesel territory are trains to Stoughton (other than parts of the Providence line that need to be electrified). The other South lines split off just after Back Bay, and half of the MBTA system is North. Either the track to Stoughton could be electrified, or they could run diesels just for those trains. My point is not that any diesel should ever touch the NEC, but that trains that run only on the NEC should be utilizing the electric infrastructure, since they are the ones that have the most time to mess up schedules for other trains. Another point where diesels run, but is out of the control of Amtrak are the Danbury Line runs to GCT, which run diesel over the New Haven Line. However, those are in MN territory, are capable of 110 operation, and run Express after they platform at SoNo. Note I said "where possible". That means MBTA Providence Line, SLE, and MARC Penn Line (which is half and half)
No, the diesels do not "mess up the schedule" for the electric trains. When it comes to the P32AC-DMs, only the locomotives are certified for 110 mph, not the cars, and you're still acting like Metro-North's going to build Class 8 track.
Elsewhere in the world, there is also extensive diesel operation through electrified territory. If the matter of SLE and MBTA continuing diesel operation in territory that is totally electrified, I remind you that that territory has only been electrified for a decade thus far and for the states of Connecticut and Massachusetts to scramble to buy specialized rolling stock may be asking too much. Same goes for MARC, especially in the case where flexibility may be necessary — diesels can run under the wire on the Penn Line, but electrics can't run on the Camden and Brunswick lines.
The Amtrak trains that switch to diesel at Philadelphia are doing so because they don't have enough electrics. That's going to end when the ACS-64s come in
No, that's not why they do it. Philadelphia is where the lines to Washington and Pittsburgh split; keeping crews at Harrisburg and Washington (two locations) instead of at just Philadelphia (one location that can service all of the trains in question) would be the greater expense. Don't expect that to change when ACS-64s show up; besides, those motors are going to replace some of Amtrak's present electric motive power rather than augment the present fleet.
You're not going to hit the same time performance as the new alignment would by just revamping the NEC, and there is a significant amount of congestion on the NEC that would be eased by taking the fastest of the fast trains off of the NEC and onto their own tracks, part of which will justify building the new line
Augmenting the Northeast Corridor to the degree it can be can result in triple-digit average speeds even approaching 110-120 mph. The expense of building a parallel NEC just to increase average speeds to 135 mph, over such a short distance, is anything but worth it.