Outside of the obvious NEC, Midwest, and California bits (and anything further is outright ASB), what would be most helpful here in this early stage is to drop the whole reticence towards foreign systems, and instead choose to learn from them. While the US isn't going to spend money to develop new HSR infrastructure (not when the national highway system is all that and a cupcake to planners - as well as the initial design standard for the Interstates specifically precluding sharing space), it could be possible to spend money to upgrade the existing network, and it's from here that it's possible to learn from other systems.
As far as the Shinkansen goes, that's going to be somewhat problematic because of hangover attitudes from WWII WRT Japan,
but it could be possible to learn how to make it work otherwise (what Japan would ultimately call IOTL "mini-Shinkansens"). Otherwise, the US rail companies and/or USDOT can do some sort of fact-finding mission overseas (even if it means coming across Italy's HSR experiments) on how to design modern passenger rail networks that coexist with highways, including visiting such exotic places as, say, Australia (where Sydney's commuter rail network at the time was, in part, inspired by New York City's subway as it existed in the early 20th century, believe it or not). While HSR is the jewel in the crown, even if at first it partially occupies a similar status as
the TEE across the Atlantic (which essentially is what the Metroliner became IOTL before the Acela came along), what would be helpful is if the feeder systems - the commuter, regional, and medium-distance intercity trains - also received similar levels of investment and modernization.
Or, IOW, WI alongside:
US governments in the 70s decided they wanted this to work out and expand, where would HS-US have been brought to and where would it link up?
our *alt-Amtrak is also essentially a Congressionally-approved interstate compact that also runs the commuter trains for Boston, New Haven/Springfield, NYC, Philly, Baltimore, Washington, the Midwest, and Buffalo/Niagara Falls, as well as regional rail services along the West Coast, the wider Northeast, and the Midwest, at least as mini-SBB/CFFs with a fare policy and railpass system similar to the Japanese? In that case, there's also possible growth for incubating other HSR services in those regions, such as Seattle, WA-Vancouver, BC, as well as some acceptance of psuedo-HSR intercity services like CN Rail's/VIA's
Tempo railcars. (Furthermore, in the case of Greater Boston, as an analog to OTL Amtrak's connecting motorcoach services, I could also potentially see a case being made for handing over
the Steamship Authority, the ferries between Providence and Newport in Rhode Island as well as the Block Island Ferry, and
the Boston Harbor commuter ferry service to *Amtrak.)