Explore the world of tank cars.

Tank Car 101


The first rail tank cars were simple, wooden containers mounted atop flat cars. They carried liquid products across America. Modern tank cars are constructed of fine grain heat treated steel and incorporate a variety of safety features to improve their puncture resistance.

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Types of Tank Cars


General Purpose tank cars are non-pressure cars that can be used to carry almost any type of material. These materials can be regulated or non-regulated commodities. The DOT-111 is the most commonly known general purpose tank car. In flammable liquids service, the DOT-111 is being replaced by the new DOT-117 standard cars. AAR-211s are the AAR equivalent of the DOT-111 standard and are typically used in non-regulated service.

Pressure tank cars–designated by the DOT-105 and DOT-112 standard specifications–are used to transport liquefied compressed gases under pressure, as well as some low-pressure, high-hazard materials.

What Does a Tank Car Carry?


From food products to clay slurry, chemicals to crude oil, tank cars are a workhorse of the American economy, bringing vital products to market. There are two general types of commodities that are transported: non-regulated and “hazmats,” which are hazardous materials regulated by the U.S. Department of Transportation.

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Rail Safety


Railroad accidents involving tank cars carrying hazardous materials occur at very low rates. More than 99.99 percent of all railroad shipments of hazardous materials reach their destination without incident. While train accidents are rare, most are attributed to just two causes: human factors or track issues (see link). When accidents occur, the safety features of tank cars help mitigate the effects, contain product and reduce the chance of release.


Non-Accident Releases (NAR)


The vast majority of tank car spills involve relatively small quantities of product–or “lading”–and are due to the car not being properly secured after loading or unloading. Shippers and railroads work hard to reduce the possibility that these Non-Accident Releases, or NAR, spills can occur. Reducing product loss helps increase safety and reduces possible environmental effects. More information on NAR is available on AAR website. Most non-accident releases occur in transportation when cars are either moving or not moving, but are due to the car not being properly secured during loading and unloading.




Tank cars are highly technical mechanical devices that have many unique characteristics. Engineers have developed a way to talk about tank cars that often include acronyms and words not found in common speech. Check here for definitions of some of the less well known terms.

AAR — the Association of American Railroads is a trade association whose membership includes freight railroads that operate over 80% of the line-haul mileage, employ approximately 95% of the workers. The AAR, among other activities, issues specifications and rules regarding the interchange of railcars between the various railroads. The AAR’s requirements for tank cars are contained in the AAR Manual of Standards and Recommended Practices (MSRP) C-III, Specification M-1002.

AAR Interchange Rules — AAR Safety and Operations staff manages the freight car Interchange Agreements and associated rules and standards. Railroads and private car owners subscribe to the AAR Interchange Agreement. The Interchange Agreement promotes railroad safety and efficient interchange. This agreement is a contractual agreement by which the parties signing agree to abide by the AAR Rules of Interchange.

AAR Specification Tank Car — a tank car built, altered, or converted in accordance with the tank car specifications of the Association of American Railroads.

A-end — the end of a railcar, opposite the end equipped with the hand brake (see B-end).

Ambient — the temperature and barometric pressure of the local environment.

B-end — the end of a railcar on which the hand brake is attached. If both ends of the railcar have a hand brake, the car will be stenciled “A-END” and “B-END.” When facing the B-end, the sides of the railcar are identified as the “right side” and “left side,” respectively.

Body Bolster — the structural members at each end of a car body that support the car on its truck assemblies.

Bottom Outlet Valve — a valve located in the bottom of the tank for loading or unloading.

Bottom Washout — a plugged and flanged opening in the bottom of a tank to facilitate cleaning of a tank car that does not have a bottom outlet.

Brake Rigging — the assembly of cylinders, levers, and/or rods under a railcar that provides and transmits braking action to the wheels.

Breather Vent — a device with an operating part that is a permeable disc or a disc with a breather hole or slit. Breather vents are typically applied to tank cars transporting hydrogen peroxide to allow vapors created by the decomposition of the commodity to be vented from the tank.

Burst Pressure — the value of the inlet static pressure at which a rupture disc device or breaking-pin device functions.

Burst Pressure (Tank) — the internal pressure at which a tank will theoretically burst. For a tank car tank, the minimum burst pressure is based on the tank’s inside diameter, welding joint efficiency, minimum tensile strength of the plate material, and the minimum thickness of the plate after forming.

CANUTEC — acronym for the Canadian Transport Emergency Centre. CANUTEC is operated by the Transport of Dangerous Goods Directorate of Transport Canada and provides a national bilingual (English and French) advisory service, including emergency response advice for incidents involving dangerous goods. Call collect at 613-996-6666 (24 hours) or *666 cellular (Canada only). Call 613-992-4624 (24 hours) for non-emergency inquiries.

Capacity/CAPY — the volumetric capacity (stenciled in gallons and/or liters) of a tank car’s tank. Capacity may also be shown in imperial gallons.

Center Sill — the center longitudinal structural member of a car underframe that forms the backbone of the underframe and transmits most of the buffering shocks from one end of the car to the other (see Stub Sill Tank Car).

Check Valve — a valve that automatically closes to stop the flow of liquid or vapor in one direction. A spring-loaded check valve operates in the closed position held by a spring; it is opened by the valve above it.

Cryogenic Liquid Tank Car — a vacuum-insulated tank car, consisting of an inner alloy (stainless) steel container (tank) enclosed within an outer carbon shell (tank, not jacket), designed for the transportation of refrigerated liquefied gases, such as liquid hydrogen, oxygen, ethylene, nitrogen and argon. These cars are built to the specifications contained in Subpart F, Part 179, 49CFR, for Class DOT-113 tank cars; Section 8.6 of TC14877E for Class TC-113 tank cars; and Chapter 3 of the AAR Specifications for Tank Cars for Class AAR-204 tank cars. Cryogenic liquids are defined by:

  1. S.: 49CFR: “A refrigerated liquefied gas having a boiling point colder than -130F (-90C) at atmospheric pressure”; and

Canada: TC TP 14877E: “A refrigerated liquefied gas that is handled or transported at a temperature equal to or less than -100°C (-148°F).”

DOT — acronym for the Department of Transportation, which is the governmental department that regulates the transportation of hazardous materials within the United States.

DOT/TC Specification Tank Car — the U. S. Department of Transportation or Transport Canada specification to which a tank car was built, altered or converted.

Double-Shelf Coupler/Coupler Vertical Restraint System — a railcar coupler with top and bottom shelves designed to prevent vertical disengagement of mating couplers in the event of an excessive speed end-to-end impact or derailment. Double-shelf couplers are fully compatible with all other railcar couplers and are required by DOT regulation on all DOT specification tank cars and any tank car transporting hazardous materials/dangerous goods.

Eduction Line — the combination of the eduction valve and adjoining eduction pipe.

Eduction Pipe — the pipe that runs from the eduction valve into the tank.

Eduction Valve — a valve used to load or unload liquid product or to introduce or remove vapor from a tank car tank.

Excess Flow Valve — a device installed in a liquid, vapor or sample line or a gauging device rod designed to stop the outward flow of product in the event the fitting is removed during transportation, such as being sheared off during an accident. When not in operation, the device allows the flow of liquid or vapor in two directions. Not to be confused with a check valve, which is a device that allows the flow of liquid or vapor in only one direction.

Expansion Dome (Dome) — a cylindrical metal enclosure located on top of an obsolete non-pressure tank car tank intended to function as the expansion area for the lading during transportation. Not to be confused with an expansion dome with protective housing, which is found on pressure tank cars and some non-pressure tank cars.

Fill Hole — an opening in the manway cover, closed with a fill-hole cover, through which product may be loaded or unloaded. Typically found on tank cars in sulfuric or hydrochloric acid service.

Fitting a pressure-retaining part, made of one or more pieces, that is in contact with the lading and has no operating components, , and that joins service equipment to the tank car tank or joins two pieces of service equipment.

Flange — a disc-shaped device that is part of a nozzle or fitting (valve, PRD) and is used to create a bolted attachment to the tank car. Also, it may be a solid disc (blind flange) or have a threaded hole for a plug or secondary valve bolted to the eduction valve to provide closure.

Gasket — material inserted in the joint between two mating surfaces to prevent leakage through the joint.

Gauging Device — a piece of equipment or a device used to measure the level of liquid or vapor space in a tank car tank. It may be a fixed gauge bar/outage scale or T-bar attached to the top of the tank below the manway nozzle (in non-pressure tank cars), or a magnetic-ball, fixed-length (telltale) tube, or an electronic device.

Hand Brake — a device mounted on railcars and locomotives to provide a means for applying brakes manually without air pressure. Common types include vertical wheel, horizontal wheel and lever type, so-named because of the configuration or orientation of their operating handles.

Head — one of the ellipsoidal ends of a tank car tank.

Head Shield — a metal shield on the end of a tank car to protect against punctures from the coupler of another railcar. Head shields may be separate attachments or may be incorporated into a tank’s jacket.

Heel — common term for the product or residue remaining in a tank car tank after it has been unloaded.

Insulation — a material, typically fiberglass or foam, enclosed within a metal jacket, used to maintain or moderate the temperature of the lading during transportation. For cryogenic liquid tank cars, in addition to a vacuum and insulation system, either perlite or an alternating wrap of multiple layers of aluminum foil and paper is used. Not all tank cars are insulated. Not to be confused with thermal protection.

Internal Valve — a type of bottom outlet valve located inside a tank car tank to prevent damage in the event of an accident.

Jacket — a metal covering (minimal thickness of 11 gauge) surrounding a tank car tank designed to protect and secure the insulation and/or thermal protection systems on a tank car. A jacket is not an outer tank.

Lading — the commodity being transported.

Light Weight/LT WT — the empty weight or tare weight of a rail car. The light weight is stenciled in pounds and kilograms on every rail freight car and is abbreviated LT WT.

Liquid Eduction Line — a pipe, equipped with a valve, cap, or blind flange closure, that extends to the bottom of a tank car tank for loading and unloading the lading.

Load Limit/LD LMT — the maximum weight of lading that can be loaded in a railcar. Load limit is stenciled in pounds and kilograms on every rail freight car and is abbreviated LD LMT.

Manway — a general term designating the circular-shaped opening located at the top of a tank car tank to allow access into the tank’s interior for maintenance, inspection, and loading or unloading. Depending upon a tank car’s class or product service, the manway will be closed with either a hinged and bolted manway cover (typical for non-pressure tank cars) or a semi-permanently bolted manway cover or pressure plate (typical for pressure and cryogenic liquid tank cars).

Nitrogen Blanket/Nitrogen Pad — nitrogen gas inserted into a tank car tank to provide an inert atmosphere for a lading that may react with air in order to protect the lading’s purity or to prevent the entry of moisture.

Non-pressure Tank Car — a tank car with a tank test pressure of 60 or 100 psig, built to the specifications contained in Subpart D, Part 179, Title 49CFR or Chapter 3, AAR Manual of Standards and Recommended Practices, Section C-III, Specification M-1002. Non-pressure tank cars are also referred to as “general service” or “low-pressure” tank cars in the 2016 Emergency Response Guidebook.

Normalized Steel — a steel plate that has been heated and held at elevated temperatures (usually 1600 – 1700F) followed by still-air or forced-air cooling. Normalization is a kind of heat treatment thatrelieves stress on steel and improves ductility and toughness.

Nozzle — a circular or oval-shaped attachment applied to openings in a tank that facilitate for the application of a manway cover (or pressure plate), valves, pressure relief devices, and other fittings.

Outage — the vacant space left in a tank car tank after filling to allow for product expansion during transportation so it will not reach shell-full capacity (maximum volume of a tank). Governmental regulations prescribe minimum outages for hazardous materials/dangerous goods at specified reference temperatures. In addition to minimum outage by volume, tank cars may not be loaded by weight in excess of their gross weight on rail limit as determined by their truck capacity. Another term for “outage” is “ullage.”

Packing — a general term denoting the various substances and devices used to prevent leakage of fluids or gases through openings (valve body and valve stem) that cannot be closed by ordinary contact of the parts concerned.

Packing Gland — the portion(s) of a device used to contain packing on a valve body or other fitting to prevent leakage.

PIH/TIH (Poison/Toxic Inhalation Hazard) — a gas or liquid that meets the definition of a “material poisonous by inhalation” as defined in §171.8, Title 49 CFR (see TIH).

Pressure Plate — on a pressure tank car, the circular-shaped steel plate closing the manway nozzle to which the valves, pressure relief device(s), and other fittings are mounted. Some non-pressure tank cars may be equipped with a pressure plate assembly in lieu of a hinged and bolted manway cover.

Pressure Relief Device (PRD) — a fitting that opens at a predetermined setting to reduce the pressure within a tank car tank that results from exposure to abnormal conditions. PRDs may be reclosing (spring-loaded) pressure relief valves, regulating valves, non-reclosing rupture disc devices/safety vents, or a combination device (incorporating both a rupture disc/breaking pin and a reclosing pressure relief valve).

Pressure Relief Valve (PRV) — a reclosing spring-loaded device, actuated by inlet static pressure, that relieves excess pressure and recloses after normal conditions are restored. “Pressure relief valve” has replaced the term “safety relief valve.”

Pressure Tank Car — a tank car with a tank test pressure of 100 to 500 psig built to the specifications contained in Subpart C, Part 179, Title 49CFR.

Protective Housing — on pressure and some non-pressure tank cars, a heavy, circular steel housing that surrounds the fittings to protect them in the event of an accident and/or unauthorized access. Non-pressure tank cars may be equipped with light-gauge steel protective housings (referred to as combination housings, breadboxes or mailboxes) that provide weather and tamper protection. Not to be confused with an expansion dome as found on some (obsolete) non-pressure tank cars. The protective housings for cryogenic liquid tank cars are boxes or cabinets on the sides or end of the tank car.

Regulating (Regulator) Valve — a reclosing (spring-loaded) pressure relief device applied to tank cars transporting certain refrigerated liquids (e.g., carbon dioxide and argon) to maintain internal pressure below a certain point. If the pressure exceeds the specified point, the valve will open and release vapor, which results in auto-refrigeration, thereby lowering the product’s temperature and pressure.

Reporting Mark and Number — the alphabetic initial stenciled on the sides and ends of every freight car to identify the railroad or private car line that owns the car. Reporting marks are assigned by the Association of American Railroads and are  typically 3 or 4 letters ending with an X (for non-railroad -owned cars), followed by 1  -6 numerals; e.g., BOEX 2017).

Residue — the amount of commodity remaining in a tank car after it has been unloaded to the maximum extent practicable. Also referred to as “the heel.”

Rupture Disc — the operating part of a rupture disc device/safety vent. When used in combination with a spring-loaded reclosing pressure relief valve, the device is called a combination pressure relief device “Rupture disc” replaces the term “frangible disc.”

Rupture Disc Device — a non-reclosing pressure relief device actuated by inlet static pressure and designed to function by the bursting of a rupture disc. These devices are also referred to as “safety vents” by DOT and TC. (See Safety Vent.)

Safety Relief Valve — See Pressure Relief Valve. “Pressure relief valve” has replaced the term “safety relief valve.”

Safety Systems — thermal protection, insulation, tank head puncture-resistance, coupler vertical-restraint and other systems used to prevent discontinuities and protect service equipment (e.g., skid protection and protective housings).

Safety Vent — a non-reclosing pressure relief device utilizing a rupture disc. The term is synonymous with “rupture disc device” (as used by AAR) and is a term used by DOT and TC. (See Rupture Disc Device.)

Sample Line — a pipe (typically 1/4 inch in diameter), equipped with a control valve, that extends to near the bottom of a tank car tank for drawing a sample of the lading.

Shell — the cylindrical section of a tank car tank without heads. Not to be confused with jacket. (See Tank Car and Head.)

Service Equipment — equipment used for filling, sampling, emptying, venting, vacuum relief, pressure relief, heating (if internal to the tank), lading temperature measurement, or measuring the amount of lading within the tank. Commonly referred to as “valves” and “fittings.”

SETIQ — acronym for the Mexican Emergency Transportation System for the Chemical Industry, a service of the National Association of Chemical Industries (ANIQ). Responders in the Mexican Republic can call SETIQ (24 hours a day) at 01-800-00-214-00. In Mexico City and the metropolitan area, responders can call 5559-1588. Elsewhere, responders should call +52-55-5559-1588.

Shipping Paper (Hazardous Materials/Dangerous Goods) — a shipping order, bill of lading, manifest, waybill or other shipping document serving a similar purpose and containing the information required by governmental regulations.

Skid Protection — a device attached to the bottom of a tank car to protect the bottom outlet, washout and/or sump (referred to as bottom discontinuities) from damage in the event of a derailment.

Specification — the specific designation within a tank car class (e.g., DOT-111A100W2).

STC Code or STCC — abbreviation for Standard Transportation Commodity Code, which is a 7-digit freight description coding system used by the North American railroad industry. For hazardous materials/dangerous goods, the STCC is referred to as the “hazmat code” and begins with 49 or 48 for hazardous wastes. The hazmat code may be found on shipping papers and may be used to access computer-based emergency response information.

Stenciling — a term used to describe the specific information that is required to be marked on the exterior surface of a tank car. Stenciling covers all forms of lettering on cars, regardless of the actual method of application.

Stub Sill Tank Car — a tank car design with draft sills at each end of the tank that utilizes the tank as part of the car structure instead of a continuous center sill. (Also see Center Sill.)

Stuffing Box — the portion of a top-operated bottom outlet valve assembly through which the valve operating rod passes to the exterior of the tank. The stuffing box contains packing that, when compressed by the packing gland nut, forms a seal around the rod to prevent leakage and keeps the rod from vibrating. The stuffing box cover, when removed and inverted, is used as a wrench to open and close the internal valve.

Sump or Siphon Bowl — a small depression located near the longitudinal center of a tank bottom where the liquid eduction line extends, thereby allowing the maximum amount of product to be removed from the tank.

Surge Pressure Reduction Device — a device designed to reduce the internal surge pressures of the pressure relief devices. These devices are primarily used on tank cars equipped with non-reclosing pressure relief devices (safety vents) and are intended to reduce pressure surges that can cause the rupture disc to fail.

Tank Car/Tank Car Tank — a railcar  with a tank for its body that consists of a shell and heads together with connections welded directly to it and is used to transport liquids, solids and liquefied gases. In accordance with AAR specifications, “tank” means “tank car tank.” The head of a tank is one of the end closures. Tank cars may be pressure or non-pressure and are often equipped with special equipment to enhance their usefulness for handling specific commodities. For pressure class tank cars, the tank includes the manway nozzle as well. Note: “Tanker” or “tanker car” are inappropriate terms to describe a tank car.

Tank Test Pressure — the pressure (psig) at which a tank car tank is to be hydrostatically tested at the time of construction. Depending on the specification, the tank test pressure varies from 20 – 40 percent of the minimum burst pressure. Tank test pressure is also known as “service pressure.”

TC or Transport Canada — the governmental agency that regulates the transportation of dangerous goods within Canada.

Thermal Protection — a material or system applied to tank car tanks to limit the transfer of heat to the tank in the event of exposure to pool or torch fires. It is intended to reduce the likelihood of tank failure under such conditions. Thermal protection is not the same as insulation, which is intended to maintain or moderate lading temperature under ambient conditions. Per 49CFR, a thermal protection system must have sufficient thermal resistance to prevent release of any lading from a tank car except through the pressure relief device when subjected to:

A pool fire for 100 minutes and/or

A torch fire for 30 minutes.

Thermometer Well — a small diameter pipe, usually 3/4 inch, filled with an antifreeze solution or oil that extends into the tank and is closed at the top with a removable cap. The temperature of the lading transfers to the liquid in the pipe, and a thermometer or probe is lowered into the pipe to obtain the lading’s temperature.

TIH/PIH (Toxic/Poison Inhalation Hazard) — a gas or liquid that meets the definition of a “material poisonous by inhalation” as defined in §171.8, Title 49 CFR (see PIH).

Train Consist — for purposes here, a document (also referred to as a “train list” or “wheel report”) that sequentially lists the location of each railcar in a train. A train consist may also serve as the shipping paper for a railcar containing hazardous materials/dangerous goods, provided it contains all of the information required by governmental regulations.

Truck — the assembly of wheels, axles, roller bearings, springs, side bearings, side frames and bolsters that supports each end of a railcar and enables it to move on the rails.

Type (Tank Car) — for tank cars, designates the approving authority (DOT, TC or AAR). Preferred usage is, for example, “DOT tank cars.”

UMLER — The Universal Machine Language Equipment Register (UMLER) is a computer platform managed by Railinc, a private company based in Cary, NC. The UMLER database is used by railroads, rolling stock owners and repair shops to share a wealth of rail-car information. It allows them to efficiently interchange cars, pool traffic and issue blocking requests.

Vacuum Relief Valve — a spring-loaded valve mounted at the top of some non-pressure tank cars, designed to open and allow air into the tank if an excessive vacuum is formed that may cause the tank to collapse. A vacuum relief valve should not be depressed to determine if there is pressure in the tank. As doing so may dislodge the sealing component, causing the device to leak vapor or liquid.

Valves — A device attached to a tank car tank designed to allow the flow of lading into and out of the tank. Purposes include, but are not limited to, measuring fluid pressure and temperature, sampling fluids in the tank, detecting or determining liquid levels, or relieving over-pressures for the purposes of emergency relief or temperature control.

Vapor Line — a pipe equipped with a valve, cap or blind flange closure that extends to the top of the tank through which vapor is introduced or removed during loading or unloading. On a non-pressure tank car, this device is usually called an “air line” and is used to introduce compressed air or vapor or an inert gas to unload the car.

Vapor Space — the space in a tank above the liquid; may also be referred to as “outage.”