As with plumbing, in logistics you have to follow the crap back to the clog, in the case of the United States it is Chicago.
From Industrial History - "The most dangerous crossing in the world."
That picture is from 1910 and the multiple tracks no longer cross at grade but Chicago is still the place where the great clogs occur, meaning that even if there were enough drivers to drive enough trucks from the California ports to the railheads, there is still the enormous problem of the congestion in Chicago.
