I've ridden a lot of Amtrak and it's never been on a single track line. Are you sure that's an actual problem? Long delays, sure, but is that the reason? Seems to usually be train equipment failure.
Having ridden a lot of Amtrak almost makes it less likely that you'd have experienced the single track problem, since quality rail experiences in the US are pretty concentrated such that someone who has good train experiences probably lives/works in an area where that is the rule. On the East coast, pretty good service arguably extends as far south as Richmond, but past that sitting on the tracks is very common. And with that you get the classic downward spiral of crappy transit. Ride a train in SC or GA, and you'll see that anyone with the money for a plane ticket has abandoned rail there.
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.