Well here's a topic that hasn't been covered in a while. Why not revive it?
I see there is logic in replacing diesel trains with high-speed electric trains, even if US track isn't suited for high speed trains. It would make sense to begin replacing trains whenever it can be done and operate them at standard speeds when the grade of track demands it, such as with the Rocky Mountains. After that obstacle is cleared, then a new grade of track could be laid for high-speed rail operations. As new track is layed, these trains would simply have more routs to travel with less restriction on their speeds.
There are also possible options for diesel trains to be provided with overhead wire attachments to have electricity provided by the power grid while in high-speed operation. Most US trains are diesel electric, so that means they could act independent of a power grid or they can have it provided by another power plant. Since most American track don't have those overhead wires, it would make sense to keep using diesel electric trains, but provide them with a means to make use of those wires when they are in place.
Maglev unfortunately has no method of transitioning, since it is a very different kind of system than HSR.