The program ended in 2020. It was a lopsided arrangement with most of the advantages of lounge sharing accruing to AGR Members, so much so that AGR members had more United lounge usage rights than United Mileage Plus members. It worked fine when it was a Continental deal because Continental did not have any overseas lounges worth talking about, and many fewer domestic lounges. Compared to that United has a vast network of lounges. And all that United passengers got to use were the NEC Amtrak lounges in exchange.
The actual code share was minuscule on Amtrak, only between EWR and Wilmington, Philly, Stamford and New Haven. No codeshare to Washington DC, Baltimore, New York, Providence or Boston. That was it. And the fact that no Acela stopped at EWR made it that much less attractive at least notionally.
Frankly there was very little in it for United Customers (except for rapidly accumulating AGR points on United flights I suppose. Not sure what restrictions were there on that) and quite a bit more for Amtrak customers. I don't think too many people really shed tears or miss it when it went away. Basically United let it lapse when it came up for renewal at the end of the previous contract period.
Here is the old FlyerTalk page on that relationship...
https://www.flyertalk.com/forum/uni...codeshare-faq-including-zfv-philadelphia.html