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

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.