JohnnyReb gave a great description, the other things accomplished by putting the visible '53' on the van trailer are (1) it prevents time-consuming mistakes in trailer-on-flat-car (TOFC), aka intermodal, operations, and also (2) speeds things up at the loading dock for palletized or sized cargo, so the crew knows what will fit from the start and doesn't waste time and energy screwing around trying to put five quarts into a one-gallon jug.
