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But the USA is VAST. Freight trains are LONG. Not much of America's West is actually double track. And when you have a shorter Amtrack and a long freight train the Amtrack loses. The Amtrack has to wait until the freight train has passed.
As the freight trains run on PSR [1] there is no fixed time as to when they pass and thus the Amtrack can not run reliable.
As such it is easier for me to drive from West Los Angeles to Las Vegas, or even to drive to LAX and then fly to LAS than it is to take the train.