In addition to maintaining Shore Line East's electric fleet, the plan proposes $40 million to purchase additional M8 cars to expand service between the New Haven Line and New York and $100 million to "upgrade Amtrak's existing catenary system and power."
That article is implying that SLE wont get M8s until Kawasaki builds more M8s. Remember M8 order was finished this month and there aren't any options left. I am thinking that those future M8s will be SLE specific and designed/approved by Amtrak engineering, not MN engineers like the existing M8 fleet. Which means the existing M8 fleet will never be certified for Amtrak. IIRC the original plan was to just retire M2s, not M4s and M6s. I suspect it was decided that the M8s wont be made Amtrak compatible and the already built M8s intended for SLE became unintentionally stuck on NHL, allowing enough M8 cars to retire the M4/M6s. NY State (owner of MN) has no reason to waste NY taxpayer money on Amtrak compatibility (until PSAS capital funding), it is solely CDOT's pet project.
The M9 order closed the M8 assembly line. The M9s are intended for ESA's opening. PSAS can't happen until after ESA opens, with ESA's M9s being delivered. At that point the M9 order is done, and Kawasaki's plant will build M10s for SLE and PSAS.
So what issues would a M10 address over a M8?
-correct voltage changing
-dual mode shoes (only some of the M8s have switchable shoes, other have the old shoes)
-LCD screens with ads/route map (fixes the greenwich/rye/sono/nh semi-express problem)
-official rollout wifi (experiment on M8), maybe only on SLE CDOT fleet
-triplets (S cars are part of the married set instead of current setup), more bike room/seats, SLE and MN disagree on bike policy
-tables, a SLE/CDOT thing that old NE RRs say is a not invented here west coast invention
A M10 might be a Kawasaki SL, with 25 hz transformers and no 3rd rail shoe, with through running at Penn (or NJT sunnyside storage). SLE doesn't need shoes, PSAS doesn't need shoes. West side Hudson line would remain dual mode diesel, since MN will never go 100% electric. Making a M10 with 25 hz and no 3rd rail is the easy political way out. No expensive NIMBY ROW construction that most voters except the protesting ones dont see, more jobs for "Buy NY" procurement rules. It is easier to get money for new cars than for other improvements, or improvements to existing cars. A M10 might also be bought by MARC if Amtrak forces them to go back to electric on the Penn line or face slot cuts. SLE's Mafersa cars were rebuilt by Kawasaki, and the M10 contract will probably require M10 to be parts compatible with M8 for maintenance.