I could see that for an at-capacity double track line like the NS Chicago line, most all trains would be forced to run at current of traffic speed. However, for a single track line, where an Amtrak train runs once a day, if it can get over a route 10% quicker, then there would be 10% less time that it would be interfering with freight traffic.
Faster Amtrak on a single track line would clog it up faster, as the freights would have to clear sooner - proably one siding sooner - thus decreasing the capacity of the line. You cannot run a single track railroad with a double track mentality.
If you are going to build a High-Speed Rail System, all of the mainline should be built for passenger trains operating at the speed of 120 miles per hour.
The engineering and construction of the tracks has to be great enough for a reasonable commute time and a safe trip at 120 Miles Per Hour.
Although 120 mph would be a big improvement, it is far from HSR as it is defined.
The difference in costs for Class 4 track (60 freight, 80 passenger) and Class 5 track (80 freight, 90 passenger) is substantial. While the increase in ties and other hardware is not that significant, the cost of the track geometry (alinement and surface) is significant. Going to Class 7 track (110 mph) basically means that you open the bank.
A larger factor is the VMax equation (49 CFR, Part 213.57) which determines speeds on curves. It doesn’t take much curvature to really limit the train speeds. You can speed trains up by using a different Eu (the elevation provided by the train) but this means special equipment testing, so freight trains will pretty much not gain any speed advantage. In fact, they will probably see more costs as the low rails will wear faster in curves due to the difference in train speeds.
A very large cost is energy. Once a train gets above 30-40 mph, extra speed means huge increases in fuel consumption. This is due to the energy curve needed for the extra speed as well as the wind resistance that a train experiences.