AAR car types

I freely use the AAR car classification to describe car types, and a recent e-mail made me aware that there are those who either don’t know about them, or don’t entirely understand them. Accordingly, in this post I provide a brief summary. It is understandable if a modeler says “I can tell a box car from a tank car, so why on earth would I bother with all this complication?” I’ll try and answer that after describing the AAR classes (or, as they are sometimes called, AAR mechanical designations).
     In the back of most issues of the Official Railway Equipment Register or ORER are the definitions of all classes, with sketches to clarify the more complicated ones, so you can always look there to get the idea or to check a code you don’t recognize. This post is only intended as an overview.
     Most cars in general service had a terminal letter “M” as in XM, TM, SM, FM, etc. (supposedly M = merchandise).  So in the absence of other information, I would guess one of the “M” categories for a particular car. But there are exceptions. Gondola classes had a bunch of complications but the most common classes are GS for drop-bottom cars and GB for tight-bottom mill gondolas. Here is an example of the ORER sketch definition for the GS cars (there are altogether 13 sketches of different gondola classes in my 1953 ORER copy, though many of them are fairly unusual cars.)

And here is part of the GB description:

     Tank cars which were insulated with an outside jacket have an “I” after the code, such as TMI. Pressurized tank cars, with the distinctive bonnet instead of a dome, are TP and usually TPI. The ORER has a long table identifying these classes, along with the also-required ICC classifications. Thus for example, a tank car might be AAR Class TM, as well as class ICC 103, or AAR Class TPI and also class ICC 105A300. In addition to the information in the ORER, more about these tank car classifications and their cargoes is available in an appendix to Ed Kaminski’s book, Tank Cars, American Car & Foundry, 1865-1955 (Signature Press, 2003).

     Hopper cars had even more variations than gondolas (there are 15 sketches in the 1953 ORER), but for many purposes one can rely on the two most common categories: twin cross-hoppers are HM, triples are HT. “Special” cars have codes beginning with “L” and that includes covered hoppers as class LO. Depressed-center flat cars are FD and TOFC cars are FC, while most other cars are FM, including heavy-duty cars, as long as they have flat decks.
     There are a lot of box car classes. In the ORER, you will see box cars identified by a number of other codes than XM. The key to understanding these is to note whether there is an “M” in the class designation. When you see a class such as XAP for auto parts racks or XAR for automobile racks, that tells you that neither of these is usable for any other loading, which is why the “M” is missing. Also present are variations on XM, such as XMR and XME. These have “stowable” racks which can be secured out of the way and then can be used for other loading. Boxcar bodies with tanks for liquids inside are actually tank cars, and have the classification XT. 

     And by the way, even though the AAR did define any house car with double side doors as an “automobile car” in 1953, all boxcar-type house cars were classified together as various X classes, without directly distinguishing box and auto cars. Indeed, most railroads in 1953 rostered part of their double-door cars in general service and classed as XM. This is one example of why you might want to know AAR classes for your models: is that double-door car in automobile service or not? The AAR class shown in the ORER will tell you.

     I will illustrate this with an example entry from the Southern Pacific pages in the 1953 ORER. Here are lines 21 and 22 of page 442 (you may wish to click on the image to enlarge it):

This entry, like many in the ORER, shows a “main listing” with a following entry line for cars which are exceptions to the main listing. The main group here is shown as AAR class XML, cars with automobile loaders. As is commonly done, the exceptions are shown in a note at the end of the SP pages, in this case Note SS. Here it is:

The note identifies the seven cars cars in the number group which are AAR class XM and thus do not have loaders. This might affect how a model would be used, depending on which car number the model carries. So this aspect might affect your choice of car numbers when decaling a model: do you want an XM or an XML (to use this SP example) for your car fleet? 

     Ice bunker reefers were RS, and those were the most common refrigerator cars in the transition era. An RS car equipped with meat rails was Class RSM, and that would the classification of many though not all meat-packer cars. “Bunkerless” insulated cars were RB, which applies to some insulated box cars as well as to cars which carried beer or beverage cargoes. Mechanical refrigerator cars were class RP (apparently P = power generation on board).
     In making out waybills or car sleeves, I always look up any car I’m not sure about. Since the codes were essential to car usage, they are present in the ORER for every car, and often you have to look at the notes at the end of a railroad entry to find out which cars had which class, as in the example above.

     So to address the question raised at the beginning of this post (which could be paraphrased as “Why should I care?”), I would say that this is another aspect of prototypical car usage and movement. Anyone wishing the car type to match the needs of a particular load or shipper on a waybill will want to use the ORER entries to find out the actual car type corresponding to a model car number. That simply enriches the prototypical quality of your freight car operations.
     Small sideline note: I just realized that this is my 101st post. I never thought I would have very much to say, when I began this blog, and was even afraid it would be a lot of work. Instead, it’s been fun, and I’m surprised I’ve already posted 100 times. Yet another amazing facet of the Internet.

Tony Thompson