What makes you think that there will be no airport-like security...
...or that it will be at "a fraction of the cost"?
Last time I looked, analysis of what the California HSR project will cost to build and operate, a one-way ticket would have to be priced at over $400 just to break even. That same ticket on Southwest goes for about $100, and gets you there twice as fast, even with airport security.
And what makes you think that the TSA will not expand to make train travel just as miserable as plane travel? After all, notice that it's called the "Transportation Security Administration", not just the "Airport Security Administration". That was for a reason.