Compare that to the 49+ million people in the northeast which would take about the same rail to cover and 17 million really is small. The problem is it's a triangle which means the maximum distance someone travels is minimized while still needing a lot of rail. You could make it a little shorter by making the trips take longer which kind of defeats the purpose. Further, there is not a lot to directly connect to it.

However, once the northeast networks expands eventually adding Texas would be reasonable, just not from day 1.

You can leave Austin and San Antonio out and still have 13 million people on the Dallas/Houston straight line.

And keep in mind that the terrain between Dallas and Houston is basically all flat and sparsely populated, so building rail there would be a lot cheaper than the northeast.

Rail only going to a fewe small city's would be underutilized. Ballpark 1/3 of the people going from A to B using high speed rail and you need a lot of people going from A to B. However, if B is on the way to C and people from A and B also go to C... etc you start to see massive network effects.

Sure, if Texas wants to spemd their money go for it, but this would be a relative ware of federal funds.