There were other rails which ran between NYC and Boston which have since been shuttered, but were far more efficient. Problem is that there isn't a populated, straight line between NYC and Boston. So the curved rail road won out due to where people live. But that's not even the real problem. The real problem is that passenger trains do not have right of way on these lines. That belongs to the freight companies. So, on average, even Acela ends up averaging 65-70mph.