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Discovered today that UA code share segments do print as e-tickets. (You can even get the PDF sent to you if you call and ask; I don't have an iPhone so I couldn't try the app route but seems likely that works, too.)
One point to note is that you can only use the lookup by UA flight number the first time; after that, you have to use the Amtrak PNR to reprint.
I have an upcoming UA reservation with two Amtrak code share segments. Exactly how does one "claim" the e-tickets in advance? Call Amtrak, call UA? Can this be done online (self-service)? I'd prefer to have my tickets in hand before leaving for the station.