I would like to preemptively suggest that the author is focusing on dense areas of the country. He is suggesting high speed rail between Portland-Seattle, Dallas-Houston, NYC-DC, etc. Not Los Angeles-Chicago or DC-Seattle.
Portland-Seattle is already a terrible candidate. I use to travel between the two once a month for a year. The Amtrak was empty as hell. If Amtrak didn't get as much in subsidies as it did to keep that going, it would have shut that shit down long ago. I never road the ORCA for Seattle-Tacoma, but that station near the amtrak always looks pretty busy during commute times.
Build small parts of the network first
Larger portions later
Exactly, start with linking neighbor cities where the density supports it. Once the infrastructure and expertise is in place, then it starts to make sense to link the existing lines together into a bigger network.