The boxcar has always been one of the key elements of every mixed freight train whether it runs as a mainline, branch line, or peddler freight.  It has been used to carry a wide variety of  freight for decades on our nation's railroads.  It has gone from the plain to the colorful billboard, to the uniform Tuscan, Boxcar Red, or Mineral Brown color, with some exceptions of course.  I started looking at the 40 foot box cars in my collection, and decided to do some research on box cars in general to determine what the manufacturers have produced, and whether they are correct for the time period I model (1950's Steam to Diesel).
    Early boxcars were originally made of wood.  The familiar wood sheathed box car lasted well into the 1950's with the last ones being built in 1941.  Wood was used during the 1940's because of a steel shortage during World War II. The first steel boxcar was actually built in 1923. It had riveted ends and a 3 panel sliding door. Many of these early cars had "truss rods" to give them more support and strength.  The truss rod car was outlawed on all cars in interchange service by 1952. Your trains from 1920, 1930, and 1940 can have cars with truss rods, but after that only your home road cars may have them.
    In 1928 the Dreadnaught, or corrugated end, was introduced.  This had more inherent strength and was better at absorbing the blows from shifting cargo than the plain ends.  These cars from this time period had the brake wheel mounted vertically which had to be accessed from the roof of the car.  By 1937 many of our boxcars were being made by different shops and railroads.  Subsequently they had different heights, which added to variety on steam freight trains. All, however, had the end mounted brake wheel which eliminated the brakeman having to climb up onto the roof to apply the brake.  Some of these 1930's era box cars lasted until the 1960's because of upgrades and regular railroad maintenance.  Some of the most famous and distinctive steel box cars of this era were the Milwaukee Road horizontal ribbed cars, and the Pennsylvania cars with the distinctive curved roof sides. The brightly painted Billboard box cars were pretty much gone from the scene by 1937.
    What we call roof walks on our freight cars are actually called "running boards" by the railroads.  These were originally made of unpainted wood, and as such became very slippery in rain or ice.  By 1945 the wood running boards were outlawed and steel became the standard. The steel running boards could be found both unpainted or painted depending upon the practices of the railroad.  Most were painted to protect them from rust and corrosion.
    By 1944 we see the most modern of the AAR or standard steel box car being manufactured.  Of the many companies that manufactured box cars, Pullman Standard produced a large number, which were designated as a model PS-1.  Marcia grew up in Michigan City, Indiana where the Pullman Standard factory was originally located.  Some of the original buildings still exist, and act as a small museum.  Old photographs show strings of box cars coming from the factory being switched to be sent to the many major railroads. Many of these photos have been enlarged and are on display within the old factory area. There are also wheel sets, tools, and other equipment that was used in car construction which have been preserved for display.
    Steel box cars had different end designs depending upon the manufacturer.  There were the Flat Riveted, Dreadnaught, Improved Dreadnaught, and the Pullman Standard ends.  In our model world of 40 foot box cars, Aristo uses the Improved Dreadnaught ends on their cars,  LGB uses Dreadnaught ends, and USA uses some sort of corrugated ends that don't seem to match any of the prototypes that I have pictures for.  As a note, Aristo seems to be stuck with some sort of obscure brake wheel design, whereas the other two manufacturers use one that was more common on cars for the time period.
    Boxcar doors also come in a wide variety of styles.  Besides the "plug door" versions, the sliding doors came in a 3 Panel Creco, Youngstown Corrugated, Superior 5 Panel, and Pullman Standard Pressed Diamond.  The Aristo and some USA cars use a variation on the Youngstown Corrugated door.  LGB and some USA cars use a 7 Panel version of the Superior door.  Unfortunately at this time, no manufacturer has made a double slide door 40 foot boxcar at this time, but they did exist on many railroads.  These double doors were a valuable asset when loading long loads like lumber, machinery, or furniture, and would add interest and variety to our model trains.
    Car roofs are another area of variation for boxcars.  There are the Flat Riveted, Viking Corrugated, Murphy Rectangular, Diagonal Panel, and Pullman Standard-Bow Tie.  Aristo box car roofs are all similar to the Flat Riveted type.  LGB uses the Diagonal Panel style roof.  USA uses a pseudo PS-Bow Tie which is not totally prototype when compared to photos.
    The 50 foot box car is not as new you might imagine.  Basically our train manufacturers have produced models of 50 foot cars using the same dies and molds as their 40 foot cars.  This said, the same criteria hold true for these cars as they did for our 40 foot fleet in terms of roofs, ends, doors. This manufacturing process is not without prototypical precedent.  The early 50 foot boxcars were produced in the 1940's, and in many cases, the railroads merely stretched their original plans for 40 foot box cars to the new length.
    A 50 foot boxcar has many advantages over it's shorter cousin.  They were often used for transporting automobiles, furniture, and lumber which did not load as easily into a 40 foot car.  These cars had a variety of door types for loading. Some had end doors, and some had double side doors.  Quite a few had large single side doors.   
    If you model the 1940's, you can have a variety of wood and steel cars mixed on your railroad.  Once you get into the 1940's, you can add some 50 foot box cars for interest.  Remember, truss rods are OK, but only for your home road once you get into a 1940's or 1950's modeling period.  In 1966, box cars were required to have lower brake wheels, and no access to the roof except for loading and unloading.  This means that all running boards and side ladders were removed, and end ladders were cut down. By 1974, this transition was completed.  Note that on  each boxcar, you will find a small rectangle off wood on each door, and on each end.  These are called "tack boards", and are temporary locations for information about what a car contains, the shipper, receiver, special instructions, etc.
    Hopefully some these tips will help in your selection of box cars for your railroad that will be compatible with the time period you are modeling. Basically we have our "big three" manufacturers producing 40 and 50 foot boxcars that are "close" but not totally prototypical.  A rivet counter would show some concern, but with the "10 foot rule" they all look just fine running on any railroad.      
Aristo 40 Foot Boxcar
LGB 40 Foot Boxcar
USA Trains 40 Foot Boxcar