AAR Equipment Type and AAR Mechanical Designation

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This page was last updated on April 15, 2026.

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AAR Equipment Type and AAR Mechanical Designation

The following basic car description codes apply to modern (post 1990) refrigerator cars and insulated boxcars.

Refrigerated and Insulated Box cars with Mechanical Designations of RB, RBL, RC, RP, RPL, XLI, XPI

(Compiled as part of Cryo-Trans research) (Use the Index at right)

AAR Equipment Type (4-character alphanumeric)

The AAR Equipment Type gives a general physical description of the type of equipment. (Also used by UMLER)

Examples: A616 and R660

Axxx -- Equipped box cars (after 1983)

First numeric:

0. All stock cars (added 1989)
1. Inside length less than 49’
2. Inside length less than 49’, cushioned
3. Inside length at least 49’ and less than 59’
4. Inside length at least 49’ and less than 59’, cushioned
5. Inside length at least 59’ and less than 79’
6. Inside length at least 59’ and less than 79’, cushioned
7. Inside length at least 79’
8. Inside length at least 79’, cushioned

Second numeric:

0. Mechanical designation XP, specific commodity
1. Mechanical designation XPI, specific commodity, insulated
2. Mechanical designation XF, USDA interior for foodstuffs
3. Mechanical designation XL, loader equipped
4. Mechanical designation XLI, loader equipped, insulated
5. Mechanical designation S, stock (added 1989)

Third numeric:

0. Other type of door opening
1. Sliding door, opening less than 9’ wide
2. Sliding door, opening at least 9’ and less than 11’ wide
3. Sliding door, opening at least IT wide
4. Plug door, opening less than 9’ wide
5. Plug door, opening at least 9’ and less than 1T wide
6. Plug door, opening at least 11 ’ wide Combination sliding and plug doors

A616 Equipment Type

A = Equipped box cars
First numeric: 6 (59'-79') [cushioned]
Second numeric: 1 (XPI)
Third numeric: 6 (Plug Door)

Equipment Type A616 = XPI Mechanical Designation

X = Box Car
P = Specific commodity (not general service; no load retainers)
I = Insulated

Other operators use A606 = XP without any refrigeration (super-insulated boxcar)

Rxxx -- Refrigerator cars (after 1983)

First numeric:

1. Inside length less than 49’
2. Inside length less than 49’, cushioned
3. Inside length at least 49’ and less than 59'
4. Inside length at least 49' and less than 59’, cushioned
5. Inside length at least 59' and less than 79'
6. Inside length at least 59’ and less than 79’, cushioned
7. Inside length at least 79’
8. Inside length at least 79’, cushioned

Second numeric:

0. Mechanical designation RB, no refrigeration
1. Mechanical designation RBL, loader equipped
2. Mechanical designation RS, bunker (deleted 1996)
5. Mechanical designation RPB (deleted 2001)
6. Mechanical designation RP, mechanical refrigeration
7. Mechanical designation RPL, loader equipped
8. Mechanical designation RPC, permanent containers (added 1985)
9. Mechanical designation RC, cryogen refrigeration (added 1987)

Third numeric:

0. All cars except those listed below
1. Fiberglass body (added 1997)

R660 Equipment Type

R = Refrigerator cars
First numeric 6 (59'-79') [Cushioned]
Second numeric: 6 (RP with mechanical refrigeration)
Third numeric: 0 (not Fiberglass body)

Equipment Type R660 = RP Mechanical Designation

R = Refrigerator
P = self-contained mechanical refrigeration

R690 Equipment Type

R = Refrigerator cars
First numeric 6 (59'-79') [Cushioned]
Second numeric: 9 (RC with cryogenic refrigeration)
Third numeric: 0 (not Fiberglass body)

Equipment Type R690 = RC Mechanical Designation

R = Refrigerator
C = cryogenic refrigeration

AAR Mechanical Designation (2, 3, or 4-character alpha)

The AAR Mechanical Designation is dependent on AAR Equipment Type. (Also used by UMLER)

Examples: RC, RP, XLI, XPI

RC, RP Mechanical Designation

RC - CRYX were RC (Refrigerator car refrigerated with cryogenics)

RP - CRYX and LINX are RP (mechanical refrigerator car)

RA - Refrigerator car with brine tanks used for meat and provisions
RB - Refrigerator car without brine tanks or bunkers used for beer, etc.
RBL - Refrigerator car without bunkers and with load retaining devices
RC - Refrigerator car refrigerated with a cryogen
RLO - Covered hopper car with mechanical refrigeration
RP - Refrigerator car with self-contained mechanical refrigeration
RPC - Refrigerator car with mechanical refrigeration and permanently attached container(s)

XLI, XPI Mechanical Designation

XPI - CRYX and LINX are XPI (insulated box car, without load retainers)

XPI cars are essentially insulated versions of the "XP" (Specialty) boxcar, often used to prevent condensation or freezing of sensitive products.

XF - Box car with an FDA approved white interior coating to prevent contamination of foodstuffs
XL - Box car with non-specific load retaining devices; replaces XME and XML
XLI - Box car, Insulated, Loader Equipped, with securements and/or permanently attached movable bulkheads
XM - Box car suitable for general service
XP - Box car equipped for and restricted to a specific commodity (not general service; no load retainers); replaces XAP and XMP
XT - Box car which is metal lined or encloses a tank and is used for liquids
X_I - Suffix for box car with insulation; e.g. XMCI, XMEI
__H - Suffix for car with heater; e.g. RBLH, XIH, etc.

L = Load Retainers

Interior Load Retainers (Loader Devices)

(The CRYX and LINX RC, RP, and XLI cars not equipped with load retainers.)

In the design of boxcars and refrigerated cars (reefers), a "loader" or loader device or load retainer refers to internal equipment designed to secure cargo and prevent it from shifting or collapsing during transit. These systems are often identified by Association of American Railroads (AAR) marks on the car's exterior.

Movable Bulkheads: Large internal walls that can be positioned against cargo to partition the car or hold a load in place.
Removable Cross Members: Bars that fit into side rails to brace individual sections of cargo.
Lading Strap Anchors: Fixed points for attaching ropes or belts to moor the cargo.

UMLER 2026 - Load Dividers are only applicable to Box Cars with Mechanical Designation of XP, XPI, XF, XL, or XLI

RB vs. RP Refrigerator Cars

What is the difference between an RB railroad car, and an RP railroad car?

In the world of North American railroad classification, the difference between an **RB** and an **RP** car comes down to how the car maintains its temperature. Both fall under the general category of refrigerator cars (the "R" prefix), but they use fundamentally different methods to protect their cargo.

RB: Bunkerless Refrigerator Car

An **RB** car is essentially a heavily insulated boxcar designed to maintain the temperature of the goods loaded into it without the use of a mechanical cooling unit. It relies entirely on high-efficiency insulation. It does not have a mechanical refrigeration unit, or ice bunkers.

Purpose: These are used for "controlled temperature" loads that need to be protected from extreme heat or freezing, but don't require a specific, constant sub-zero temperature.
Typical Cargo: Beer, canned goods, wine, and certain grocery items that just need to stay cool or stay above freezing during transit.
Key Identifier: Because there is no mechanical equipment, these cars lack the external refrigeration unit visible on the end of the car and do not have fuel tanks underneath.

RP: Mechanical Refrigerator Car

An **RP** car is a fully "mechanical" refrigerator car equipped with an engine-driven cooling and heating system. It features a diesel-powered mechanical refrigeration unit (often made by companies like Carrier or Thermo King) that can actively lower or raise the interior temperature to a specific thermostat setting.

Purpose: These are used for perishable goods that require strict climate control over long distances, particularly frozen foods or fresh produce.
Typical Cargo: Frozen meat, frozen vegetables, ice cream, or fresh fruits that require constant airflow and specific humidity/temperature levels.
Key Identifier (older designs): In older designs from the 1950s through the 1970s, the refrigeration unit was mounted on one end of the car, protected by a grill, and a large diesel fuel tank mounted under the car's frame.
Key Identifier (newer designs): In newer designs beginning in the early 2000s, you will see a mechanical refrigeration unit very similar to those on highway trailers, mounted to the end of the car.

Comparison Table

Feature	RB (Bunkerless)	RP (Mechanical)
Cooling Source	None (Insulation only)	Diesel-powered mechanical unit
Heating Source	None (Insulation only)	Active mechanical heater
Primary Goal	Temperature stability	Temperature control (Precision)
Complexity	Low (essentially an insulated box)	High (requires fuel and maintenance)
Common Use	Beverages, canned goods	Frozen foods, fresh produce

A Note on Modern Usage

In modern railroading, the **RB** designation has largely been superseded by the **RBL** designation (the "L" indicates the car is equipped with load-restraining devices like damage-prevention bulkheads). Most "insulated boxcars" seen today are technically RBLs, but the core principle remains: they are passive containers compared to the active mechanical systems of the **RP**.

XLI Insulated Box Cars vs. RB Refrigerator Cars

An **XLI** railroad car is a specialized evolution of the standard boxcar, sitting somewhere between a traditional freight car and an insulated refrigerator car. While the "R" series (like the RB and RP) focuses on refrigeration, the "X" series denotes various types of boxcars.

XLI: Insulated Boxcar, equipped with load restraining devices

XLI is a boxcar that has been specifically built or modified with **insulation** and **interior loading equipment**, but it is technically classified as a boxcar rather than a refrigerator car.

The "X" (Boxcar): It is fundamentally a boxcar designed for general freight.
The "L" (Equipped): This indicates the car is fitted with permanent interior "loading equipment," such as adjustable bulkheads, side rails, or "DF" (Damage-Free) bars to secure the load.
The "I" (Insulated): This signifies that the walls, roof, and floor are lined with insulating material to protect against sudden temperature fluctuations.

How an XLI Car Differs from an RB Car

While an **XLI** and an **RB** might look almost identical from the outside, the distinction lies in their primary classification and intended service:

The **RB** is a "Bunkerless Refrigerator" car. It is built to the standards of a refrigerator car and is primarily intended for shipments that require a very high thermal efficiency to keep things cool.
The **XLI** is an "Insulated Boxcar." It is intended for general merchandise that simply needs to be kept from freezing in the winter or overheating in the summer (like paper products, certain chemicals, or dry groceries), while the internal hardware (the "L") keeps the cargo from shifting.

Summary of Characteristics

How it Works: Like the RB, the XLI is a **passive** system. It uses thick insulation to buffer the interior from the outside environment. It has no engine, no fuel, and no ice.

Purpose: To carry "clean" freight that requires both protection from the weather and secure bracing to prevent physical damage.
Typical Cargo: High-end paper, canned food, bottled water, or moisture-sensitive industrial products.
Key Identifier: They look like standard modern boxcars but usually have much thicker, "plug-type" doors. Unlike standard sliding doors, plug doors pull out and slide sideways, creating a flush, airtight seal when closed to maintain the interior temperature.

Quick Comparison

Feature	XLI (Insulated Equipped)	RB (Bunkerless Reefer)
Primary Category	Boxcar (X)	Refrigerator (R)
Internal Bracing	Always (The "L")	Optional (Becomes RBL)
Insulation Level	Moderate to High	High
Door Type	Almost always Plug Door	Always Plug Door
Main Function	Damage & Temperature protection	Temperature stability

XLI vs. XPI Insulated Box Cars

XPI: Insulated Equipped Boxcar

An **XPI** railroad car is a highly specialized variant of a boxcar that combines insulation with a specific type of interior configuration designed for heavy or specialized loads. Like the XLI, it is classified under the "X" (Boxcar) category rather than the "R" (Refrigerator) category.

The **XPI** designation identifies a car that is **Insulated** ("I") and equipped with **Permanent Partitions** ("P").

The "X" (Boxcar): It is a member of the general boxcar family.
The "P" (Partitions): This indicates the car features permanent or semi-permanent interior dividers or bulkheads. Unlike the "L" (Equipped) designation which often refers to removable bars or rails, the "P" usually denotes heavy-duty, movable bulkheads that lock into place to compartmentalize the car.
The "I" (Insulated): The car body is fully insulated to protect against outside temperature extremes.

Functional Role

The XPI is designed for the high-density transport of goods that are both sensitive to temperature and prone to shifting. The internal partitions allow the loader to "section off" the car, preventing a heavy load at one end from sliding and causing a derailment or damaging the cargo at the other end.

Summary of Characteristics

How they Works: XLI and XPI cars are a **passive** thermal car. They use high-grade insulation to maintain the temperature of the cargo at the time of loading. They lack any mechanical cooling or heating units.
Purpose: XLI and XPI cars are is built for heavy-duty, "damage-free" transport where climate stability is a secondary but necessary requirement.
Typical Cargo: Large shipments of bottled or canned beverages (which are heavy and prone to freezing/bursting), heavy industrial chemicals in drums, or bulk paper products.
Key Identifier: Externally, XLI and XPI cars are look like large, modern boxcars with **plug doors**. Internally, they are characterized by heavy steel bulkheads in the XPI cars that hang from ceiling tracks and can be rolled into position to snug up against the face of the cargo. The XLI cars use removable rails and bars to restrain the load.

Comparison Table

Feature	XLI (Insulated Equipped)	XPI (Insulated Partitioned)
Commonality	Insulated boxcar with plug doors.	Insulated boxcar with plug doors.
Loading Gear	Uses "L" equipment (bars, side rails, or straps).	Uses "P" equipment (large movable bulkheads).
Load Type	Generally lighter, palletized merchandise.	Heavier, high-density loads requiring bracing.
Door Style	Plug Door.	Plug Door.

Classification Context

In many modern equipment registers, the distinction between **XLI** and **XPI** can become blurred as many cars are dual-rated or the "L" designation is used as a catch-all for any load-restraining device. However, technically speaking, if you see the "P," you are looking for those heavy-duty internal wall dividers specifically designed to prevent longitudinal shifting of the load.

UMLER

UMLER (Universal Machine Language Equipment Register) is the comprehensive, centralized electronic database for North American rail equipment, managed by Railinc for the Association of American Railroads (AAR). It serves as the primary registry for over two million pieces of equipment, tracking physical characteristics, maintenance, and ownership for railroads, shippers, and equipment owners.

UMLER is the authoritative source for data used in railcar interchanging, pooling, scheduling, and safety compliance.
UMLER includes details on railcars, locomotives, and trailers, such as dimensions, capacity, weight, and inspection history.
UMLER is used by industry professionals (railroads, owners, agents) to manage equipment, including adding, updating, and viewing the status of assets.
It acts as a foundational file for other railroad industry systems and is essential for effective interline operations.

The Universal Machine Language Equipment Register (UMLER) system was established in 1968 to manage North American rail equipment data. It began as a system that used IBM punch cards and computer tapes, to Electronic Data Interchange (EDI) that connected railroads and car owners. In 2009 it was transitioned to today's modern Umler/EMIS system to improve communication for car owners through the use of a modern electronic database. The current updated system was launched by Railinc in July 2009.

While the UMLER system started in 1968, detailed historical lineage for equipment is only available from 1992.

Overall AAR Mechanical Designation System

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