The loaded cars can go coast to coast, but trains are broken up and reorganized often in yards, especially in places like Chicago. this is efficient because two cars from LA headed to Toronto and NYC can travel together until Chicago and only then be split into two new trains. Even within a single railroad this is commonly done.

A train leaving Seattle isn't loaded entirely with cars going to Philadelphia. Some are going to Miami, some are going to Texas, some at going to Chicago. There might even be cars in Chicago that get picked up by that train on its way to Philadelphia.

Nobody is talking about taking boxes out of one car and putting them on another, but it seems pretty self-evident that the entire train itself would get broken up multiple times on a cross-country trip.