If you want real ridership, public transit in both LA and LV needs to be functional.
Connect it to Union Station and LAX if you want some ridership. On the Las Vegas end make it stop smack in the middle of the strip.
Otherwise this thing is useless.
Also 125mph is not high speed rail. LA-LV in 2 hours + time spent getting to the train station + buffer time to avoid missing the train + long slow car rental line on the other end while 15 people ahead of you each have a chit chat with the one employee dealing with the rentals would be more time than just driving end to end. These people just don't get it.
Try LA-LV in 1 hour and maybe we're talking. But really, fix public transit in both cities first.