I have no specialized knowledge in this, but I don’t think that cost efficiency is the primary reason for using trucks over trains. Because, if it was, rail with last mile by truck would probably win. Rail has a very low cost per mile by weight rate. But, it can be slow. Particularly when you consider that there are only a few routes across the country (across the Rockies), and trucks can go directly from point A to point B without needing to connect to a rail hub at both ends.

In someways it seems similar to why the hub and spoke model has fallen out of favor for airlines. It might be more efficient to have one large flight between hubs and two small flights to regional airports, but passengers would rather have a direct flight.