You can probably squeeze about a half an hour out of the current DC-Boston train time without spending a dime on infrastructure, simply by running trains more efficiently (especially in western Connecticut where I'd swear they're dispatching intercity trains behind the local commuter train given how pokey it goes).

Outside of that, there's probably about 100 miles of track that can support 220mph speed limits in current right-of-way (large sections in MA, MD, and RI). Adopting better trainsets (switch to EMUs from locomotive-hauled) and rebuilding track to higher speed limit standards would help there. The track from about Kingston, RI to New Rochelle, NY basically needs dedicated HSR. You can, for the most part, use the existing I-95 ROW for this, and east of New Haven, there's basically no takings you have to consider; west of New Haven, you need to do a lot more takings for curve straightening, and that is going to be an eminent domain disaster.