AAR Mechanical Designation

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This page was last updated on April 15, 2026.

(Sources include a wide variety of internet searches, including current and abandoned websites at Archive.org. The focus of this page is to use sources not previously readily available.)

AAR Mechanical Designation

Carl Shaver described the overall AAR Mechanical Designation System:

There has been a system of Mechanical Designations for freight cars used for nearly 100 years. For most of its history, it's been governed by the Association of American Railroads (AAR); prior to that it was the Master Car Builders’ Association (MCB).

In this system, each basic freight car type is designated with a letter. Variations of these cars are covered with one or two other letters in most cases. Here are the basic car types you’ll see nowadays (others are now obsolete):

X - Box car
R - Refrigerator car
T - Tank car (all tank cars just carry the "T" designation now)
G - Gondola car
H - Hopper car
F - Flat car
L - various special car types.

Beyond those letters, you need a source like the Official Railway Equipment Register (ORER) to provide the current designations. They don't change often, at least not drastically.

Concerning flat cars. A normal flat car (unequipped) is an FM. If it's modified to carry a specific commodity, it's an FMS. If it's equipped with an auto rack, it's an FA. An FB has bulkheads, and an FBC is a Center-Beam car, of the type now commonly used for lumber. An intermodal flat car, for trailers or containers, is designated FC; if it's articulated it's designated FCA.

Here are a couple more that you'd probably see a lot of:

GT or GTS: A high-sided gondola usually used for coal transportation.
HT: A plain hopper car, with three or four bays beneath (two bays is HM).
HTS: Generally refers to the quick-dumping coal hoppers, but could be for any hopper designed to carry a specific commodity. Woodchip hoppers, for example, are also HTS.
LO: covered hoppers.
GB: a standard gondola.
XM: a standard plain box car, no special interior equipment
XL: A box car equipped with interior load-restraining devices
XP: A box car equipped to transport a specific commodity (and generally unsuited for other commodities).
XMI, XLI, XPI: Same as above, only insulated.
LC: A box car with roof hatches (generally used for bulk products like clay).
LU: An all-door box car (very rare nowadays).

The AAR Car Type Codes will further break these down into length, size and type of door, etc.

The AAR Mechanical Designation also applies to passenger cars with several classifications.

P - Passenger-carrying car
B - Baggage and express
M - Mail (usually RPO)
D - Diner

AAR Mechanical Designation Car Codes (ORER 1965)

(View a PDF document taken from the 1965 Official Railway Equipment Register, ORER -- Shows all of the Mechanical Designation codes at that time.) (PDF; 13 pages; 122KB)

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