Acela is technically capable of speeds that would be considered "high speed" for a legacy system (which is what this is, it's not like they could tear out New York city and build fresh then put it all back) of over 150mph. Unfortunately lack of investment means it doesn't achieve what is possible along most of the route.
Two main things you'd do if you had money and political will to speed this up:
1. Tear out old bridges (some have been "urgently in need of replacement" for a decade already) and replace with modern ones. Locals like how the old bridge looks? Great, hand over the money for a new one that looks the same. No money? You get a generic reinforced concrete bridge rated for 120mph and suck it up. This can be done in 24 hours per bridge for a short bridge, shut the railway, smash the old bridge, drop the new one in, test it, re-open the railway. If you have the manpower you can do ten bridges at once.
2. Eliminate at-grade crossings. These are unsafe even for lower speeds, they're lethal for High Speed Rail and must be closed. If practical add a bridge or tunnel. If not, too bad, close them anyway and people have to go round.
> it's not like they could tear out New York city and build fresh then put it all back
Several large cities in Europe have built new high speed tracks in tunnels under the city.
The track between London St Pancras and Stratford stations in London (and from Stratford further east) is an example. St Pancras is the end of the line, so the speed is necessarily limited by the acceleration and braking capability of the train, but [1] says the speed is 230km/h = 142mph. [2] says speeds in tunnels are limited to 270km/h (168mph), they probably mean a bit further out of the city.
Other than the last 1km or so, the whole track of this route is new.
Realistically, any city worth spending so much money on will have every train stopping at it, so there's no need for higher speeds in under-city tunnels.
> which is what this is, it's not like they could tear out New York city and build fresh then put it all back
I don't understand why not? Japan's high speed rail was added into multiple major cities without tearing up the whole city. What did they do that can't be done in the US?
Two main things you'd do if you had money and political will to speed this up:
1. Tear out old bridges (some have been "urgently in need of replacement" for a decade already) and replace with modern ones. Locals like how the old bridge looks? Great, hand over the money for a new one that looks the same. No money? You get a generic reinforced concrete bridge rated for 120mph and suck it up. This can be done in 24 hours per bridge for a short bridge, shut the railway, smash the old bridge, drop the new one in, test it, re-open the railway. If you have the manpower you can do ten bridges at once.
2. Eliminate at-grade crossings. These are unsafe even for lower speeds, they're lethal for High Speed Rail and must be closed. If practical add a bridge or tunnel. If not, too bad, close them anyway and people have to go round.