Desert Xpress (now XpressWest) did their due diligence for a privately run high speed service to Vegas. Unfortunately, true high speed (150mph or higher) costs real money (denominated in $x10e9), which involves politics in the real world because private funding can't deal with the long-term payback horizon of HSR, and the No-HSR-anywhere (aka Evil Thomas) people prevented them from getting a construction loan. Oh well.

The UP line between LA & LV simply isn't suitable for even low-end high speed (90-125mph), like Brightline is doing. Too many hills/curves/freight trains. Best you could do is a revived Desert Wind, which died for good and proper (read: lack of ridership and revenue, abetted by infrequent/inconvenient/unreliable schedules) reasons.

Brightline is much shorter (on the order of a state-funded Amtrak service) with a very specific and well-researched ridership and revenue base (tourists between Miami and Orlando); on basically level ground; using a rail line (in part) that's already aligned suitably for higher speeds and only needed modest improvements; where freight service is under the same control as the passenger service and is already tightly scheduled (so more like Europe than UP/BNSF lines in the desert); and targeting the service design and equipment for what's probably the "sweet spot" in the US - high end conventional/low end high speed. It will still be interesting to see how the project works out, but they do seem to be working through their issues (yes, politics did intrude, but mostly on a local level) and should be able to start service when they expected to. Consider it a test case for partial privatizing of Amtrak, if similar conditions can be identified elsewhere for "cherry-picking."