I don't want to get into the weeds with this recurring argument, but a few points:
1. While overall project costs between the two are similar, the feds will likely be picking up the tab for half of D2 v less than 10% for Cotton Belt (though the rest is financed by very low interest loans from the feds).
2. While projected new ridership from the D2 project is less than for the CB, it's still more than half, not basically zero (I mean, you can say they're both basically zero, but that's another discussion). The Commerce alignment of the old at-grade proposal was estimated at 6,600 net new rail riders.
3. Regardless, new ridership isn't so much the point of D2, whereas it is the sole reason for CB. Resilience/redundancy, operational flexibility, decreased travel times for existing riders, increased potential capacity.. D2 is pretty good at improving what it's meant to improve, while CB (with new ridership as primary goal) is not. Also, CB is exactly the kind of project (low-density, auto-oriented, brand new line) where ridership models tend to drastically over-estimate ridership.
4. The snark implying that redundancy isn't that valuable because US-75 doesn't have redundancy is bizarre. It has huge, totally underutilized arterials running parallel on both sides of it, not to mention another limited-access highway running parallel to it less than 3 miles to the west.
Here's what Greenville looks like for its entire length north of Mockingbird:
And on the other side, here's what Hillcrest looks like north of Northwest Hwy:
And then there's Preston, and Skillman, and Abrams..