Isn't a new tunnel needed to lay more tracks to increase train frequencies to Palmdale from LA?

Tunnels are not cheap, ask Amtrak, NY, and NJ how expensive tunnels can be.

Please take the time to watch this video, at least scan through it quickly.

https://www.youtube.com/watch?v=Pwh9WLHqHgk
The video from 2004 is old enough to not even have the Palmdale Station on it, but it does reflect the grade and curvature of this rail corridor. I'm finding it very difficult believing anyone wishing to take a HSR train to LV over it, or using Metrolink to catch a Virgin train in Palmdale. Lancaster to Los Angeles in one hour and 43 minutes, over a distance of 76.6 miles, is only averaging 45 mph. Today trains are scheduled to take over 2 hours and 7 minutes to make the same trip, averaging 36 mph today.

https://en.wikipedia.org/wiki/Antelope_Valley_Line
Some quick math:

76.6 miles / 103 minutes x 60 minutes / hour = 44.6 mph

76.6 miles / 127 minutes x 60 minutes / hour = 36.2 mph

Ever wonder why Amtrak refuses to run trains on this corridor as is?

FYI, Los Angeles elevation is 285 feet, Lancaster elevation is 2359 feet. That's an elevation difference of 2074 feet. only two building in the world stand as tall or taller; Burj Khalifa at 2,717 feet and Shanghai Tower at 2,073 feet.