There is some discussion of that in this thread: http://www.flyertalk.com/forum/conti...ak-co-faq.html (look around post #200 and later).

I believe I've also seen some discussion of it in the Amtrak forum, although not in awhile.

My fuzzy recollection of the subject is as follows:

In the days of yore, Amtrak relied on paper tickets, as they did not have any e-ticketing solution in place. With paper tickets, Amtrak had no way to check if you were on the train in real-time and wouldn't know until they processed the paper tickets a day or two or three (or more) later. Thus, as long as you printed your paper ticket (which generally could only be done at PHL, although there was some discussion of trying to convince Amtrak ticket agents at other stations to print it with some computer system trickery--I can't recall how successful those attempts were), there were no issues with your outbound itinerary at EWR. However, there was a distinct risk of your return being canceled if Amtrak found out that you were not on your original outbound leg before you returned home. That's probably not an issue on an immediate turn on a mileage run, but if you were gone for a week or more, you could probably assume your reservation had vanished.

That was then, and this is now. As far as I've seen, the subject hasn't come up in the couple of months since e-ticketing went live around the nation, but suffice it to say that Amtrak can now know immediately if people no-show for their reserved trains. In fact, Amtrak's system is programmed (like most airlines) to automatically cancel subsequent Amtrak segments (in a multi-train itinerary) if you no-show for your train. It remains to be seen whether Amtrak's system will communicate the fact that you've no-showed to United's system quickly enough to prevent you from taking your flight, but at this point, I would not suggest risking it.