For the definite answer on UA Club access to Amtrak lounges:
- From now until February 4, 2021, United Club Members with a valid United Club Card, including their spouses and children under the age of 21 or two guests, and United Polaris Business Class passengers with a flight coupon or boarding pass with a same-day, international segment in United Polaris Business Class are entitled to access lounges in Washington - Union Station, Philadelphia - William H Gray III 30th Street Station, New York - Penn Station and Boston - South Station
I had thought now that I'm dual-homed in DC that in more normal times I may have been able to leverage the Union Station lounge in some way but alas...