I'm not sure how true that is even though Amtrak is one mover of people. One of the reasons I for one tend to take Amtrak is that Boston area into Manhattan is mostly a pretty awful drive and it isn't because there are so few vehicles on the road. Also, pre-Acela business travelers mostly flew (and many still do) on the northern section of the corridor. And, even today, most business travelers aren't going to take the train from Boston to DC.

So it seems pretty probable to me that Acela came about because there was already a huge and increasing demand for such a service.