The current plan is NJ Transit going to a new terminal underneath/near the current Penn to clear up space for Amtrak. ...
Other work they are planning on doing is expanding the # of tracks between the end of the Hudson tunnels and near Newark.
No. No no no.
NJ Transit is constructing a new ROW alongside Amtrak's, IIRC diverging at Secaucus Junction. It involves replacing Portal Bridge with a pair of new bridges, one much higher and fixed, one slightly higher and movable, but to be moved less often. One of them is NJTs, and the other is Amtrak's- although I forget which.
The new NJT right of way will divert 2 trains an hour during peak periods from New York Penn Station as it currently exists to a new terminal slightly removed from the current Penn Station to be called "New York Penn Station Expansion". Those two slots go to Amtrak. MidTOWN Direct trains get directed, or as Dave Allen puts it, "evicted", from NYP, to be replaced with more NEC and NJCL Long Branch trains, as well as Raritan Valley Line trains.
An additional 22 trains an hour, as well as the other two, supposedly (I personally don't think it can handle that many, but that's just me), - primarily Bay Head express trains, MidTOWN Direct trains, Express trains from west of Dover, Pascack (I think), and Main/Bergen trains- will be routed via the new right-of-way to the "deep-cavern" NYPSE.
Monyhan will not be built under this plan- there is no need for it other than to erect a monument to another man's ego- in this case the late Senator Monyhan. Basically, it would be paid for by Amtrak- at huge expense - to the benefit of NJ Transit (who isn't interested in abandoning their newly built concourse) and Long Island Rail Road who also has a pretty decent concourse- and is in the process of building another station of their own under Grand Central Terminal. It'll be built if somebody decides to gift it to Amtrak.
Ok, so Penn Station is ugly. Ok, so its not impressive. Ok, so it has capacity problems. But it does work- and in any case, Monyhan is more about moving the existing platforms a little further west and building a grand ediface over them than any meaningful capacity improvement.