HSR will have to include most if not all of the major populations centers in Texas and Oklahoma for it to make it to the Rio Grande and beyond.
You obviously haven't looked world wide for any examples of how a HSR line works.
Let's take Europe as an example.
There's a HSR line between London (population 13.6 million) and Paris (population 12 million) over 212 miles.
There's a HSR line between London (population 13.6 million) and Brussels (population 2million) over 194 miles.
In France, here are the existing HSR lines
1. LGV Sud-Est Paris (population 12 million) to Lyon (population 2.2 million) over 289 miles.
2. LGV Atlantique Paris (population 12 million) to Le Mans (population 293 thousand) over 129 miles.
3. LGV Rhône-Alpes Lyon (population 2.2 million) to Valence (population 62 thousand) over 66 miles.
Note: This line is also used by trains heading to Marseille
4. LGV Nord Paris (population 12 million) to Calais (population 72 thousand) over 146 miles.
Note: This line is also used by trains heading to both London (Eurostar) and Brussels (Thalys).
5. LGV Méditerranée Valence (population 62 thousand) to Marseille (population 1.7 million) over 115 miles.
6. LGV Est Paris (population 12 million) to Strasbourg (population 885 thousand) over 248 miles. (first phase opened June 2007)
7. LGV Rhin-Rhône Lyon (population 2.2 million) to Basel (population 830 thousand) over 181 miles. (first phase opened December 2011)
8. LGV Perpignan-Figueres Paris (population 12 million) to Barcelona (population 5 million) over 644 miles in 7 hours 25 minutes.
There's a reason the lines are split up the way they are, that's how far the trains go generally. If you're wishing to travel by HSR from Paris to Marseille, there's usually a transfer in Lyon. The lines are basically the same as the train names. The farthest one can usually ride a HSR train within France is usually less than 300 miles (Paris to Lyon)
Let's compare the city populations and distances for an I-35 line through Texas.
DFW (6.4 million) to OKC (1.3 million) over 194 miles.
DFW (6.4 million) to Waco (260 thousand) over 112 miles.
Waco (260 thousand) to Austin (2 million) over 102 miles.
Austin (2 million) to San Antonio (2.2 million) over 80 miles.
San Antonio (2.2 million) to Laredo (636 thousand) over 156 miles.
Laredo (636 thousand) to Monterrey (4 million) over 143 miles.
Using that same 300 miles limitations along I-35, let's assume the main train runs between DFW and SA where the transfers to go further north or south will occur. If you wish to travel between OKC and Laredo, you'll have to take three trains with the aforementioned transfers in DFW and SA.
What I'm suggesting is that the distances are too great to include any city north of DFW and south of SA initially. The HSR system will have to work and stand on its own merits between them. I agree heading north of DFW will probably mean connected to both OKC and Tulsa, and heading south of SA will probably have to reach Monterrey. But those would be extensions off the main DFW and SA line (train).