Railroads spend a lot of time worrying about clearances. One low bridge or a narrow tunnel will limit the kinds of cars able to move across that line. To sort out which cars can operate on a given line, the Association of American Railroads categorizes freight cars based on their outside dimensions.
The AAR cross-section drawings are called “plates.” Plate B cars, for instance, are 10′-8″ wide by 15′-1″ high and can travel on any North American line.
Freight cars have not gotten any wider over the years, but they have become much longer and taller. The bigger the car, the higher its assigned plate letter. Plate H cars, such as double-stack intermodal container cars, are 20′-2″ tall. A Plate H car creates quite a mess when it meets a highway overpass that will only clear a Plate F car, so railroads make really, really certain the listed clearances of their lines are accurate.
