A tank car is a piece of railroad rolling stock designed to carry liquefied loads, petroleum products, liquid chemicals and gases. In the United Kingdom and countries that follow their railway practices tank cars are generally called tank wagons or tanker wagons.
The first tank built in the United States was in Pennsylvania to move petroleum. It consisted of three open vats bolted to a flat car. The problem came as tank car design and liquid cargo types expanded. Investing in new cars all the time made tank cars unappealing to railways, so a new company was founded: Union Tank Car Company. this company was charged with building, maintaining and running, on behalf of the railways, the tank car fleet.
Many variants exist due to the wide variety of liquids and gases that can be transported. Tank cars can be insulated or non-insulated, pressurized or non-pressurized, and designed for single or multiple loads. Non-pressurized cars have plumbing at the bottom for unloading, and may have an access port and a dome, housing various valving on the top. Pressurized cars have a pressure plate, with all valving, and a protective cylindrical housing (dome) at the top. Loading and unloading are done through this opening.
Tank cars are specialized pieces of equipment, with the interior of the car is usually lined with a material to isolate the car's structure from the contents, such as glass. Loading a liquid into a car that is designed to carry something else is unwise and sometimes dangerous. Even after a thorough cleaning, traces of the previous contents may remain, potentially contaminating the next load. Also, loading a tank car with something it is not designed to carry may actually damage the car, for example if the contents are corrosive.
As a result of this specialization, tank cars have always 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 both directions, the limited size of the tanks made this style unsuccessful.
Because of their one-way nature, tank cars are simply dead weight half of the time, making them unappealing to major railroads. 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 interior linings and not by the cargo carried. Food service tank cars are lined with stainless steel, glass or cleanable plastic and they are marked as non-pressurized, insulated cars. Usually these are small and carry around 10,000 gallons. 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. They are insulated, usually non-pressurized cars (however, very light petrochemicals or jet fuel cars will be "padded" with nitrogen to remove the air in the vapor space). These cars are larger, around 23,000 gallons. the ends will be doubled to prevent ruptures during accidents. As of 2007, the multicargo type mentioned above is obsolete. Natural gas, LPG or ammonia carrying cars are basically 60,000 gallon alloy steel pressure vessels on steel wheels. They have no linings, and are double ended. The whale belly type is giving way to higher but standard width cars.
All tank cars are inspected periodically for damage and corrosion. Pressure-relief valves and mounts are inspected at every loading. Pressurized cars are pressure tested regularly to insure they are solid. All tank cars operating in North America now feature "double shelf" couplers that will not uncouple in an accident, so the coupler will not puncture other 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 carries liquefied argon. Cars designed for multiple loads have internal bulkheads to separate the contents. Each compartment must have separate plumbing and its own dome if so equipped. The added complexity of multiple-load cars means that they make up a small percentage of the tank car population. If two loads must be transported, it is often simpler to use two tank cars instead of a two-load car.
A milk car is a specialized type of tank car designed to carry raw milk between farms or regional creameries and processing plants. Not all milk cars were tank cars. Today, milk would be chilled before loading, and moved in a glass lined, food service tank car.
A tanktainer, also known as a tank container, is a specialized type of container designed to carry bulk liquids as with the liquid hydrogen tanktainer or dangerous goods on standard intermodal equipment. The tank is held within a box-shaped frame the same size and shape as a container.
A torpedo car or bottle car is a special type of railroad car used in steel mills to haul molten hot metal. A torpedo car is a large, torpedo or shuttle shaped vessel which rests on trunnions upon two bogies. Inside of the vessel has been insulated to endure extremely hot temperatures and to keep the metal molten - usually for at least one week. The vessel can be pivoted along its longitudinal axis to empty the hot metal into a furnace (such as basic oxygen furnace) or crucible. The torpedo cars are usually used to haul pig iron from blast furnace to primary steelmaking.
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 the modern conveniences 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, could hold 17,100 gallons (64,730 liters). The car, called the largest wooden tank car ever built, took 18 months to complete construction. Obsolete in 2007- vinegar now moved in ordinary tank cars lined with glass, plastic, or alloy steel.
In the early 1960s, the Union Tank Car Company (UTLX) introduced a series of "whale belly" tank cars which offered increased capacity over the standard cars of the day. Capable of carrying 33,000 gallons (125,000 l) (for example CSOX #31084) to as much as 63,000 gallons (238,500 l) in the case of 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 (189,200 l) gallons, and is now on display at the Museum of Transportation in St. Louis, Missouri -- first hit the rails in 1963 and remained in service for over twenty years. This behemoth is 89 feet (27 m) in length and weighs 175,000 lb. (79,400 kg) empty; the car, which rides on four two-axle trucks to distribute the additional weight, was used to transport such diverse substances as liquefied petroleum gas (LPG) and anhydrous ammonia.