I mostly rode the train from SC to Boston and back (Southern Crescent) ... it's dual track all of the way. Also DC to Chicago, southern route, dual track all of the way.
I love Amtrak and am willing to put up with the delays, but DC to Chicago stop all the time, especially in the mountains, to wait for higher priority trains to pass. Sometimes it's only for a few minutes at a time, but sometimes it adds up to hours.