I've been in & out of the DC, Philly, and NYC Amtrak lounges a handful of times over the last 4 weeks and had no issues getting access for myself or guests at any. In Philly, I show my ticket so they know to get me on the right elevator for departure, but in DC and NYC, I haven't been asked to see an Amtrak ticket with my United Club card.
And I agree with the other poster: free snacks/drinks + clean bathroom is a nice perk to have when waiting in Penn Station for a train.