Never did that yet, still seems a bit risky to me.
I gather that when you mentioned upthread that you add ZFV to your tickets you are talking about putting it solely as the last leg, then dropping it. Does anyone have experience using ZFV-EWR-XXX one way and just dropping the ZFV_EWR "flight"? About a year ago, Amtrak started actually scanning their boarding passes on the trains, and it may be that the system can better communicate with UA that you didn't take the train.
Can you print UA codeshare boarding passes for Amtrak online, or must they be printed at the station?
Additionally, has anyone been able to just get on to the train at stations closer to EWR such as Metropark?