Quick version of the question: Can I board an Acela train and then use my smart phone to purchase a ticket once on board?

Longer version of the question: I was ticketed on 179 last night, the 645p BOS-NYP departure. My meeting in the Back Bay ended early, so I went to Back Bay station to catch a South Station-bound MBTA train so I could hang out in the club. (For those unaware, you can ride BBY-BOS on MBTA trains for free. Don't know if it's official policy, but they never come around to collect tickets. Amtrak conductors often will allow you to do it too.) As I was waiting on Track 2, 2175, the 520p Acela, pulled onto Track 1. I didn't think of it at the time, but now am wondering if I could have just hopped on the train and purchased a ticket on board. I'm pretty sure amtrak.com would not sell a ticket for a train that already has departed, but I certainly would have been able to purchase a ticket RTE-NYP -- which is the same price as BBY-NYP. (And if the train happened to be sold out, I could have just hopped off at RTE and waited for my train.) So as a practical matter, this seems doable. I'm just wondering how the conductor would have reacted if she came by to collect tickets before I completed the purchase on my iphone. Would she have been troubled by this strategy?