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Many variants exist due to the wide variety of liquids and gases transported. Tank cars can be pressurized or non-pressurized, insulated or non-insulated, and designed for single or multiple commodities. Non-pressurized cars have various fittings on the top and may have fittings on the bottom. Some of the top fittings are covered by a protective housing. Pressurized cars have a pressure plate, with all fittings, and a cylindrical protective housing at the top. Loading and unloading are done through the protective housing.
Tank cars are specialized pieces of equipment. As an example, the interior of the car may be lined with a material, such as glass, or other specialized coatings to isolate the tank contents from the tank shell. Care is taken to ensure that tank contents are compatible with tank construction.
As a result of this specialization, tank cars have generally been "one-way" cars. Other cars, like boxcars, can easily be reloaded with other goods for the return trip. Combinations of the two types were attempted, such as boxcars with fluid tanks slung beneath the floors. While the car could certainly carry a load in both directions, the limited tank size made this unsuccessful.
A large percentage of tank cars are owned by companies serviced by railroads instead of the railroads themselves. This can be verified by examining the reporting marks on the cars. These marks invariably end in X, meaning that the owner is not a common carrier.
Within the rail industry, tank cars are grouped by their type and not by the cargo carried. Food-service tank cars may be lined with stainless steel, glass, or plastic. Tank cars carrying dangerous goods are generally made of different types of steel, depending on the intended cargo and operating pressure. They may also be lined with rubber or coated with specialized coatings for tank protection or product purity purpose. The tank heads are also stronger to prevent ruptures during accidents. The whale-belly type is giving way to higher-capacity, yet standard-width cars.
All tank cars undergo periodic inspection for damage and corrosion. Pressure relief valves are inspected at every loading. Pressurized cars are pressure-tested regularly to insure the integrity of the tank.
All tank cars operating throughout North America today feature "double shelf" AAR type "F" couplers that prevent disengaging in event of an accident or derailment. This all but eliminates the chance of couplers puncturing adjacent tank cars.
Insulated cars (which may also incorporate heating or refrigeration systems) are used when the contents must be kept at a certain temperature. For example, the Linde tank car depicted below[which?] carries liquefied argon. Cars designed for multiple commodities are constructed of two or more tanks (compartments). Each compartment must have separate fittings. The lower capacity and added complexity of multicompartment cars means that they make up a small percentage of the tank car inventory.
Outside of North America, tank cars are also known as tank wagons or tanker wagons. In the United Kingdom tank wagons were traditionally four-wheel vehicles. Some long-wheelbase four-wheelers are still in use but bogie vehicles are now used as well.
An old Esso Tanker, at the Nene Valley Railway, England
The DOT-111 tank car, which are designed to carry liquids, such as denatured fuel ethanol, is built to a US standard. The design has been criticised on safety grounds. The train in the Lac-Mégantic derailment of 2013 was made up of 72 of these cars.
DOT-112 tank cars are used in North America to carry pressurized gases.
DOT-114 tank cars are used in North America to carry pressurized gases.
Milk is now commonly chilled, before loading, and transported in a glass-lined tank car. Such tank cars are often placarded as "Food service use only".
A pickle car was a specialized type of tank car designed to carry pickles. This car consisted of several wooden or metal vats (typically three or four) and was often roofed. Pickles which are preserved in salt brine were loaded through hatches in the roof.
A tank container, also known as ISO tank, is a specialized type of container designed to carry bulk liquids, such as chemicals, liquid hydrogen, gases and foodgrade products. Both hazardous and non hazardous products can be shipped in tank containers. A standard tank container is 20 feet (6.1 m) long, 8 feet (2.4 m) high and 8 feet (2.4 m) wide. The tank, which is made from stainless steel, is held within a box-shaped frame with the same shape as an intermodal container. This allows it to be carried on multiple modes of transport, such as truck, rail and ship.
The thermally-insulated vessel is mounted on trunnions, and designed to endure extremely high temperatures, as well as keeping the metal in a molten state over extended periods of time. The vessel can be pivoted along its longitudinal axis to empty the pig iron into a furnace (such as a basic oxygen furnace) or crucible. The name is derived from the vessel's resemblance to a torpedo.
A vinegar car is a specialized type of tank car designed to transport vinegar. The largest such car built was built by Morrison Railway Supply Corporation in 1968. The car's underframe included all of modern facets of freight car design including roller bearing trucks and cushioning devices built by FreightMaster, while the tank that rode on it, made of Douglas Fir, had a capacity of 17,100 US gallons (65 m3; 14,200 imp gal). The car, in what has been called 'the largest wooden tank car ever built', took 18 months to build.
In the early 1960s, the Union Tank Car Company (UTLX) introduced a series of "whale belly" tank cars which offered increased capacity. These cars carried from 33,000 US gallons (120 m3; 27,000 imp gal) in CSOX #31084 to as much as 63,000 US gallons (240 m3; 52,000 imp gal) in GATX #96500, which had been conceived as a 'rolling experiment' of sorts. The largest tank car ever placed into regular service, UTLX #83699, was rated at 50,000 US gallons (190 m3; 42,000 imp gal). It first hit the rails in 1963, remained in service for over twenty years, and is now on display at the Museum of Transportation in Saint Louis, Missouri. This behemoth is 89 feet (27.1 m) in length, weighs 175,000 lb (79,400 kg) empty, and rides on four two-axle trucks to distribute the additional weight. It was used to transport such diverse substances as liquefied petroleum gas (LPG) and anhydrous ammonia.
Where AAR (Alliance) couplers are fitted, the type "F" coupler is preferred as these are less likely to become separated in a derailment and become a puncture hazard for other tank cars. The type "F" coupler has shelves over and under the coupler that prevent the coupler sliding out of position after a derailment.
In addition, some types of tank cars have a second end plate visible from the exterior, to further avoid end punctures.
Tank cars have been involved in many rail disasters, including the Lac-Mégantic derailment in Quebec in 2013, the Nishapur train disaster in Iran in 2004, and the Viareggio train derailment in Italy in 2009.
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